# Lecture 9: Infectious Disease Pathology ## Unit 2.11: Pathology --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the general mechanisms of infectious disease 2. Explain host-pathogen interactions 3. Describe the pathology of bacterial infections 4. Explain viral infection patterns and tissue tropism 5. Describe fungal and parasitic infections 6. Explain emerging infections and host defense mechanisms --- ## Lecture Outline ### I. Principles of Infectious Disease Infectious diseases are caused by a diverse array of pathogenic organisms that can be classified into five major categories based on their structure, biology, and mechanisms of disease. Bacteria, encompassing both gram-positive and gram-negative organisms, represent the most commonly encountered pathogens and produce disease through a combination of direct tissue damage and host inflammatory responses. Viruses, which include both DNA and RNA viruses, are obligate intracellular parasites that commandeer host cellular machinery for replication. Fungi, ranging from unicellular yeasts to filamentous molds, produce disease particularly in immunocompromised hosts. Parasites, including protozoa and helminths, cause enormous global morbidity especially in tropical and developing regions. Prions, consisting of misfolded PrPSc protein, represent the most unusual category and produce invariably fatal neurodegenerative diseases without any nucleic acid genome. The outcome of an encounter between a pathogen and a human host spans a broad spectrum determined by pathogen virulence, inoculum size, and host immune status. Colonization represents the mildest interaction, in which a pathogen is present on body surfaces without causing disease, as seen with commensal organisms of the skin and gut. Infection occurs when a pathogen successfully multiplies within host tissues, though this does not necessarily produce symptoms. Disease develops when the infection becomes symptomatic, reflecting tissue damage from either the pathogen or the host inflammatory response. The carrier state describes a particularly important clinical scenario in which an individual asymptomatically sheds the pathogen, serving as a reservoir for transmission to others. Latency refers to a dormant state in which the pathogen persists in host tissues without active replication but retains the capacity to reactivate, as exemplified by herpesviruses and Mycobacterium tuberculosis. Pathogens gain access to the body through specific routes of entry, each exploiting particular vulnerabilities in host defenses. The respiratory route represents one of the most efficient means of transmission, allowing pathogens such as Mycobacterium tuberculosis, influenza virus, and SARS-CoV-2 to enter through inhaled droplets or aerosols. The gastrointestinal route serves as the portal for fecal-oral pathogens including Salmonella, Vibrio cholerae, and rotavirus, which must survive the hostile gastric environment to establish infection. The skin provides entry for organisms such as Staphylococcus aureus and human papillomavirus, particularly when the epithelial barrier is disrupted by wounds or insect bites. The urogenital tract serves as the route for sexually transmitted infections, while blood-borne pathogens including HIV, hepatitis B, and hepatitis C virus require direct inoculation into the bloodstream through needle sharing, transfusion, or needlestick injury. Transplacental transmission allows pathogens comprising the TORCH complex to pass from mother to fetus during pregnancy, producing devastating congenital infections. The capacity of a pathogen to cause disease depends on specific virulence factors that enable attachment, invasion, immune evasion, and tissue damage. Adhesins are surface molecules that mediate specific attachment to host cell receptors, a prerequisite for colonization and infection. Toxins represent potent virulence factors that produce direct damage to host tissues, with exotoxins secreted by living bacteria and endotoxins released upon bacterial lysis. Capsules composed of polysaccharide surround many pathogenic bacteria and impair phagocytosis by preventing complement deposition and opsonin recognition. Enzymes such as hyaluronidase, collagenase, and coagulase facilitate tissue penetration and spread. Biofilm formation allows organisms to adhere to foreign bodies and form structured communities encased in a polysaccharide matrix that confers remarkable resistance to both antibiotics and host immune defenses. Panel A: Classification diagram showing the five major pathogen categories - bacteria, viruses, fungi, parasites, and prions - with representative examples and structural features of each. Panel B: Spectrum of host-pathogen interactions from colonization through infection, disease, carrier state, and latency with clinical outcomes. Panel C: Routes of pathogen entry illustrated on human body showing respiratory, gastrointestinal, skin, urogenital, blood-borne, and transplacental pathways. Panel D: Bacterial virulence factors including adhesins for attachment, capsule preventing phagocytosis, toxins causing direct damage, and biofilm formation protecting from antibiotics. --- ### II. Tissue Responses to Infection The host tissue response to infection follows several characteristic patterns that reflect both the nature of the invading pathogen and the type of immune response mobilized. Suppurative inflammation, defined by the accumulation of neutrophils forming pus, represents the hallmark response to pyogenic bacteria such as staphylococci and streptococci. Granulomatous inflammation involves the organization of macrophages into compact aggregates, often with multinucleated giant cells, and characterizes the response to organisms such as Mycobacterium tuberculosis and certain fungi that resist intracellular killing. Cytopathic responses occur when viruses directly kill infected cells, producing tissue necrosis without a prominent inflammatory infiltrate. Necrotizing inflammation reflects aggressive tissue destruction by particularly virulent organisms or toxins. Chronic inflammatory patterns dominated by lymphocytes and plasma cells develop when pathogens persist despite the acute response, reflecting the sustained engagement of adaptive immunity. Bacterial infections elicit tissue responses that vary according to the organism's virulence factors and the anatomic site of infection. Abscess formation, characterized by a walled-off collection of pus, is the signature lesion of Staphylococcus aureus, which produces coagulase and other enzymes that confine the infection while resisting neutrophil killing. Pseudomembranes, composed of fibrin, necrotic epithelium, and inflammatory cells adherent to mucosal surfaces, are produced by Clostridioides difficile in the colon and Corynebacterium diphtheriae in the pharynx. Granulomatous responses with organized macrophage aggregates characterize infections by Mycobacterium tuberculosis and Treponema pallidum in syphilis, reflecting the host's attempt to wall off organisms that resist intracellular destruction. Cellulitis, a diffusely spreading infection through soft tissues, typifies streptococcal infection, where enzymes such as streptokinase and hyaluronidase break down tissue barriers and prevent the localization that characterizes staphylococcal abscess. Viral infections produce distinctive tissue responses that reflect the virus's interaction with host cells and the immune system's efforts at clearance. Cytopathic effects involve direct viral killing of host cells, producing necrosis and tissue damage proportional to the viral burden and the regenerative capacity of the affected tissue. Inclusion bodies represent accumulations of viral products within infected cells, appearing as either intranuclear or intracytoplasmic structures that are often pathognomonic for specific viral infections. Syncytia, formed when viral fusion proteins cause adjacent cells to merge into multinucleated giant cells, are characteristic of respiratory syncytial virus, measles, and herpes simplex virus. Latency represents a state of viral quiescence within host cells, as exemplified by the herpesvirus family, which establishes lifelong latent infections in neural ganglia or lymphocytes. Oncogenesis occurs when viral gene products interfere with cell cycle control and tumor suppressor pathways, as seen with human papillomavirus and Epstein-Barr virus. Granulomatous inflammation represents a particularly important tissue response pattern that occurs with several categories of infectious organisms. Mycobacteria characteristically produce caseating granulomas, in which the central area undergoes a distinctive cheesy necrosis surrounded by epithelioid macrophages and multinucleated giant cells. Fungi, particularly the endemic mycoses such as Histoplasma and Coccidioides, frequently elicit granulomatous responses that may closely resemble tuberculosis histologically. Parasites such as Schistosoma species provoke granuloma formation around eggs deposited in tissues, with the granulomatous response ultimately producing the fibrosis responsible for the major clinical manifestations of schistosomiasis. The common thread among these diverse organisms is their resistance to intracellular killing, which provokes the sustained macrophage activation and organization that defines granulomatous inflammation. Panel A: Suppurative inflammation histology showing abscess with central pus composed of neutrophils, necrotic debris, and bacteria surrounded by pyogenic membrane. Panel B: Granulomatous inflammation with organized collection of epithelioid macrophages, multinucleated giant cells, and surrounding lymphocyte collar. Panel C: Viral cytopathic effects including cell lysis, syncytia formation from cell fusion, and intranuclear inclusions within infected cells. Panel D: Comparison of different viral inclusion bodies - Cowdry A in herpes, ground-glass nuclei in HBV, and owl-eye inclusions in CMV. --- ### III. Bacterial Infections - Gram-Positive Staphylococcus aureus stands as one of the most versatile and clinically important human pathogens, producing a remarkable range of infections from superficial skin lesions to life-threatening systemic disease. Its virulence depends on a formidable array of factors, most notably coagulase, which converts fibrinogen to fibrin and helps the organism wall itself off from host defenses, and multiple toxins that directly damage tissues and modulate immune responses. The range of infections caused by S. aureus includes skin and soft tissue infections, infective endocarditis with particular predilection for the tricuspid valve in intravenous drug users, and osteomyelitis with destruction of bone. Specific toxins mediate distinctive clinical syndromes: toxic shock syndrome toxin-1 (TSST-1) functions as a superantigen causing massive T cell activation with shock and multiorgan failure, while enterotoxins produce a self-limited but dramatic food poisoning. The pathologic hallmark of S. aureus infection is abscess formation, with collections of neutrophils and necrotic debris surrounded by a fibrous wall that both contains the infection and limits antibiotic penetration. Methicillin-resistant S. aureus (MRSA) has become a major clinical concern, carrying the mecA gene encoding an altered penicillin-binding protein that confers resistance to virtually all beta-lactam antibiotics. Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes a wide spectrum of infections ranging from mild pharyngitis to rapidly fatal necrotizing fasciitis. The organism produces pharyngitis, the most common bacterial cause of sore throat, as well as cellulitis with diffuse spreading erythema through soft tissues, and the devastating deep infection of fascial planes known as necrotizing fasciitis or "flesh-eating disease." Its virulence depends on multiple toxins including streptolysins O and S, which lyse red blood cells and damage leukocytes, and erythrogenic toxins responsible for the rash of scarlet fever. The most clinically significant aspect of GAS infection is the potential for post-infectious immune-mediated sequelae: rheumatic fever, in which molecular mimicry between streptococcal M protein and cardiac tissue produces valvular damage, and post-streptococcal glomerulonephritis (PSGN), in which immune complexes deposit in the glomeruli causing nephritis. The pathologic hallmark of streptococcal infection is a spreading cellulitis rather than the localized abscess typical of staphylococcal disease, reflecting the organism's production of enzymes that degrade tissue barriers. Streptococcus pneumoniae represents the prototypical encapsulated pathogen, with its polysaccharide capsule serving as the principal virulence factor by preventing phagocytosis. The capsule functions as a potent anti-phagocytic mechanism, inhibiting complement deposition and opsonin recognition, and forms the basis for the pneumococcal vaccine. S. pneumoniae is the most common cause of community-acquired bacterial pneumonia, typically producing a lobar pattern of consolidation in which an entire lobe becomes filled with inflammatory exudate. It is also a leading cause of bacterial meningitis in adults and a frequent cause of otitis media in children. The characteristic rusty sputum associated with pneumococcal pneumonia results from blood-tinged inflammatory exudate within the alveoli, reflecting the intense inflammation and capillary damage produced by the infection. The genus Clostridium encompasses several anaerobic, spore-forming species that produce some of the most potent toxins known. Clostridioides difficile, recently reclassified from Clostridium, produces pseudomembranous colitis in the setting of antibiotic-associated disruption of normal colonic flora, with toxins A and B causing epithelial damage and the formation of characteristic yellow-white pseudomembranes on the colonic mucosa. Clostridium tetani produces tetanospasmin, which blocks the release of inhibitory neurotransmitters to cause the sustained muscular spasms of tetanus. Clostridium botulinum produces botulinum toxin, the most potent biologic toxin known, which prevents acetylcholine release at neuromuscular junctions to cause the descending flaccid paralysis of botulism. Clostridium perfringens produces alpha toxin (lecithinase) that destroys cell membranes, causing the myonecrosis and gas production of gas gangrene, a rapidly progressive and often fatal soft tissue infection characterized by crepitus and tissue destruction. Panel A: Staphylococcus aureus abscess showing central liquefactive necrosis with neutrophils, gram-positive cocci in clusters on Gram stain, and fibrous wall formation. Panel B: Streptococcal cellulitis demonstrating diffuse spreading infection in dermis with erythema, edema, and chains of gram-positive cocci. Panel C: Lobar pneumonia caused by Streptococcus pneumoniae with consolidated lung lobe, rusty sputum, and lancet-shaped diplococci. Panel D: Clostridium difficile pseudomembranous colitis showing yellow-white pseudomembranes on colonic mucosa composed of fibrin, mucus, and inflammatory cells. --- ### IV. Bacterial Infections - Gram-Negative Escherichia coli represents one of the most diverse bacterial pathogens, with multiple pathotypes causing distinct clinical syndromes through different virulence mechanisms. Enterotoxigenic E. coli (ETEC) produces heat-labile and heat-stable enterotoxins that stimulate secretory diarrhea, making it the most common cause of traveler's diarrhea. Enterohemorrhagic E. coli (EHEC), particularly serotype O157:H7, produces Shiga toxin that damages colonic endothelium causing hemorrhagic colitis and may trigger hemolytic uremic syndrome (HUS) with microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. Uropathogenic E. coli (UPEC) possesses specific adhesins including P fimbriae that mediate attachment to uroepithelium, making it the most common cause of urinary tract infections. Enteropathogenic E. coli (EPEC) causes infantile diarrhea through an attaching and effacing mechanism that disrupts the intestinal brush border. As a gram-negative organism, E. coli possesses lipopolysaccharide (LPS) endotoxin in its outer membrane, which can trigger a systemic inflammatory response leading to sepsis when the organism enters the bloodstream. Salmonella and Shigella species cause important gastrointestinal and systemic infections with distinct clinical presentations reflecting their different pathogenic strategies. Salmonella typhi produces typhoid fever, a systemic infection in which the organism invades through intestinal epithelium, survives within macrophages, and disseminates through the reticuloendothelial system to produce sustained bacteremia with fever, hepatosplenomegaly, and the characteristic rose spots on the abdomen representing bacterial emboli in dermal vessels. Non-typhoidal Salmonella species typically cause a self-limited gastroenteritis with nausea, vomiting, and diarrhea, reflecting infection confined to the intestinal mucosa. Shigella species, in contrast, produce bacillary dysentery by directly invading colonic epithelial cells, causing mucosal ulceration with bloody, mucoid stools, and abdominal cramps reflecting the intense colonic inflammation. Pseudomonas aeruginosa is a quintessential opportunistic pathogen that thrives in hospital environments and causes infections primarily in immunocompromised hosts and those with disrupted barriers. The organism is particularly dangerous in the settings of neutropenia, severe burns, cystic fibrosis, and mechanical ventilation, where it can cause devastating pneumonia, urinary tract infections, and wound infections. Its virulence depends on biofilm formation, which enables persistent colonization of medical devices and airways, and exotoxin A, which inhibits host protein synthesis by ADP-ribosylating elongation factor 2. Ecthyma gangrenosum is a distinctive and pathognomonic skin manifestation that occurs in neutropenic patients, presenting as necrotic skin ulcers with black eschar resulting from Pseudomonas invasion of dermal blood vessel walls, producing a vasculitis with thrombosis and tissue infarction. Neisseria species include two important human pathogens with distinct clinical manifestations. Neisseria meningitidis is a leading cause of bacterial meningitis and meningococcemia, characteristically producing a petechial and purpuric rash reflecting disseminated intravascular coagulation and vascular damage. Its virulence depends on pili for mucosal attachment and lipo-oligosaccharide (LOS) that triggers intense inflammation. The most devastating complication of meningococcal infection is Waterhouse-Friderichsen syndrome, in which overwhelming sepsis produces bilateral adrenal hemorrhagic necrosis leading to acute adrenal insufficiency, cardiovascular collapse, and often death despite aggressive treatment. Neisseria gonorrhoeae causes gonorrhea, the second most commonly reported sexually transmitted infection, producing purulent urethritis in men and cervicitis in women, with potential complications including pelvic inflammatory disease and disseminated gonococcal infection. Panel A: E. coli pathotypes diagram showing ETEC with enterotoxin secretion, EHEC with Shiga toxin causing hemorrhagic colitis, and UPEC adhering to uroepithelium. Panel B: Typhoid fever showing rose spots on abdomen, Peyer patch hyperplasia with necrosis, and intracellular Salmonella typhi within macrophages. Panel C: Pseudomonas ecthyma gangrenosum with necrotic skin lesions showing black eschar, angioinvasion, and gram-negative rods on stain. Panel D: Neisseria meningitidis infection with petechial rash, purulent meningitis, and Waterhouse-Friderichsen syndrome with bilateral adrenal hemorrhage. --- ### V. Mycobacterial Infections Tuberculosis, caused by Mycobacterium tuberculosis, remains one of the most important infectious diseases worldwide, progressing through distinct clinical stages that reflect the dynamic interplay between bacterial persistence and host immunity. Primary tuberculosis occurs upon initial exposure, typically producing the Ghon complex consisting of a small peripheral lung lesion combined with enlarged draining hilar lymph nodes, and is usually asymptomatic in immunocompetent individuals as cell-mediated immunity successfully contains the infection. Latent tuberculosis represents the contained but persistent state in which viable mycobacteria remain dormant within granulomas, detectable only by a positive tuberculin skin test (TST) or interferon-gamma release assay, without radiographic or clinical evidence of active disease. Reactivation tuberculosis develops when host immunity wanes, characteristically affecting the upper lobes of the lungs where high oxygen tension favors mycobacterial growth, and producing cavitary lesions that are highly infectious due to the enormous bacterial burden within cavities. Miliary tuberculosis results from hematogenous dissemination of the organism, producing innumerable small granulomas resembling millet seeds throughout the lungs and other organs, representing a particularly dangerous form that occurs in severely immunocompromised patients. The pathologic hallmark of tuberculosis is the caseating granuloma, a highly organized structure that reflects the host's cell-mediated immune response to persistent mycobacterial antigens. The central feature is caseation necrosis, a distinctive form of cell death producing a cheesy, amorphous, eosinophilic material that is essentially pathognomonic for mycobacterial infection. Surrounding the necrotic center are epithelioid macrophages, activated macrophages with abundant pink cytoplasm that form the core of the granuloma. Langhans giant cells, formed by the fusion of multiple macrophages, display the characteristic horseshoe or peripheral arrangement of nuclei that distinguishes them from foreign body giant cells. The outer rim consists of a collar of lymphocytes, predominantly T cells, that sustain the macrophage activation through interferon-gamma secretion. Over time, granulomas may undergo fibrosis and calcification, producing the calcified nodules visible on chest radiography that represent healed primary tuberculosis. Extrapulmonary tuberculosis develops when mycobacteria spread beyond the lungs through lymphatic or hematogenous routes, affecting virtually any organ system. Lymph node tuberculosis, historically termed scrofula, is the most common extrapulmonary manifestation, typically involving cervical lymph nodes with caseating granulomatous inflammation. Spinal tuberculosis, known as Pott disease, produces vertebral body destruction with potential paravertebral abscess formation and spinal cord compression. Genitourinary tuberculosis characteristically presents with sterile pyuria, reflecting the presence of white blood cells in urine without conventional bacterial pathogens growing on standard culture media. Tuberculous meningitis shows a predilection for the base of the brain (basilar meningitis), producing a thick exudate that may compromise cranial nerve function and obstruct cerebrospinal fluid flow. Miliary tuberculosis disseminates widely, producing small granulomas throughout multiple organs simultaneously, often with a fulminant clinical course requiring urgent diagnosis and treatment. Leprosy, caused by Mycobacterium leprae, represents a mycobacterial infection whose clinical manifestations are determined by the host's immune response rather than the virulence of the organism itself. Tuberculoid leprosy develops in patients with a strong cell-mediated immune response, producing well-formed granulomas with few detectable bacilli, limited and well-demarcated skin lesions, and asymmetric nerve involvement. Lepromatous leprosy occurs in patients with weak cell-mediated immunity, characterized by absent granuloma formation, enormous numbers of acid-fast bacilli packed within foamy macrophages, and diffuse symmetric skin thickening with a leonine facies. Borderline leprosy occupies an intermediate position along this immunologic spectrum, with features of both polar forms in varying proportions. A critical pathologic feature shared by all forms of leprosy is nerve involvement, with Mycobacterium leprae displaying a unique tropism for Schwann cells of peripheral nerves, producing sensory loss that leads to secondary injuries, ulcerations, and the characteristic deformities of leprosy. Panel A: Tuberculous granuloma showing central caseating necrosis with cheesy appearance, surrounding epithelioid cells, Langhans giant cells with horseshoe nuclei, and lymphocyte collar. Panel B: Ghon complex diagram showing primary lung lesion with calcified focus plus enlarged hilar lymph node representing initial TB infection. Panel C: Miliary tuberculosis with chest X-ray and gross pathology showing innumerable small granulomas resembling millet seeds disseminated throughout lung parenchyma. Panel D: Leprosy spectrum comparing tuberculoid (well-formed granulomas, few bacilli, strong CMI) versus lepromatous (poorly formed granulomas, numerous bacilli, weak CMI) with nerve involvement in both. --- ### VI. Viral Infections - DNA Viruses The herpesvirus family comprises several ubiquitous human pathogens that share the defining characteristic of establishing lifelong latent infections with potential for reactivation. Herpes simplex virus type 1 (HSV-1) typically causes gingivostomatitis as its primary infection, then establishes latency in the trigeminal ganglia, from which it reactivates to produce the recurrent cold sores (herpes labialis) that affect millions of people. Herpes simplex virus type 2 (HSV-2) predominantly causes genital herpes through sexual transmission, establishing latency in sacral ganglia and reactivating to produce recurrent painful vesicular eruptions on the genitalia. Varicella-zoster virus (VZV) causes chickenpox as its primary infection, a generalized vesicular exanthem, then remains latent in dorsal root ganglia and may reactivate decades later to produce shingles (herpes zoster), a painful dermatomal vesicular eruption. Epstein-Barr virus (EBV) infects B lymphocytes, causing infectious mononucleosis as its primary infection and establishing latency within B cells, with reactivation linked to several malignancies including Burkitt lymphoma, nasopharyngeal carcinoma, and Hodgkin lymphoma. Cytomegalovirus (CMV) establishes latency in monocytes and produces a mononucleosis-like syndrome in primary infection, but causes devastating disease in immunocompromised patients including retinitis, colitis, and pneumonitis. Herpetic infections produce characteristic pathologic findings that are often diagnostic on histologic examination. Multinucleated giant cells result from viral fusion proteins that merge adjacent infected cells into syncytia, a hallmark of both HSV and VZV infections. Cowdry type A inclusions are eosinophilic intranuclear inclusions surrounded by a clear halo created by margination of chromatin to the nuclear periphery, representing sites of viral replication within infected cell nuclei. The Tzanck smear, a rapid bedside diagnostic test performed by scraping the base of a vesicle and staining the cells, demonstrates these characteristic multinucleated giant cells and is useful for rapid presumptive diagnosis of herpetic infections, although it cannot distinguish between HSV and VZV. Human papillomavirus (HPV) infects squamous epithelium and produces a spectrum of disease ranging from benign warts to invasive cancer, with clinical outcome determined by the viral type. Low-risk types, particularly HPV 6 and 11, cause common skin warts and anogenital condylomata acuminata that remain benign. High-risk types, most importantly HPV 16 and 18, are the causative agents of cervical cancer and are increasingly recognized as the cause of oropharyngeal squamous cell carcinoma. The oncogenic mechanism depends on two viral proteins: E6, which targets the p53 tumor suppressor for ubiquitin-mediated degradation, removing the critical guardian of the genome, and E7, which inactivates the retinoblastoma (RB) protein, releasing E2F transcription factors to drive uncontrolled cell proliferation. Koilocytes, squamous epithelial cells displaying characteristic perinuclear halos with nuclear enlargement and hyperchromasia, represent the cytologic hallmark of HPV infection and are identified on Papanicolaou smears during cervical cancer screening. Hepatitis B virus (HBV) is a DNA virus that infects hepatocytes and produces a spectrum of disease from acute self-limited hepatitis to chronic infection with eventual cirrhosis and hepatocellular carcinoma. Acute infection is often asymptomatic, particularly when acquired in early childhood, but may produce jaundice, elevated transaminases, and rarely fulminant hepatic failure. Chronic hepatitis B, defined by persistence of hepatitis B surface antigen (HBsAg) for more than six months, develops in approximately 90% of perinatally infected infants but only 5% of adults, reflecting the maturity of the immune response at the time of infection. The pathologic hallmark is the ground-glass hepatocyte, in which the cytoplasm takes on a pale, finely granular eosinophilic appearance due to massive accumulation of HBsAg within the endoplasmic reticulum. Oncogenesis occurs through both indirect mechanisms, as chronic inflammation and regeneration promote the accumulation of mutations, and direct mechanisms involving integration of viral DNA into the host genome, which may activate oncogenes or disrupt tumor suppressors. Panel A: Herpes simplex infection showing multinucleated giant cells with nuclear molding, Cowdry type A eosinophilic intranuclear inclusions with peripheral chromatin margination, and Tzanck smear appearance. Panel B: Human papillomavirus-infected epithelium with koilocytes displaying characteristic perinuclear clearing, nuclear enlargement, and hyperchromasia in cervical squamous cells. Panel C: Hepatitis B virus ground-glass hepatocytes with pale eosinophilic cytoplasm representing accumulated HBsAg within endoplasmic reticulum on H&E and orcein staining. Panel D: Epstein-Barr virus mononucleosis showing reactive lymphocytes (Downey cells) on peripheral smear with irregular nuclear contours and abundant basophilic cytoplasm. --- ### VII. Viral Infections - RNA Viruses Human immunodeficiency virus (HIV) infection progresses through well-defined clinical phases that reflect the ongoing battle between viral replication and the host immune system, ultimately culminating in acquired immunodeficiency syndrome (AIDS). The acute phase occurs within weeks of infection, producing a flu-like illness with high viremia as the virus replicates explosively in the absence of a specific immune response. The latent or chronic phase follows, during which the viral load reaches a set point maintained by the adaptive immune response, but a gradual and relentless decline in CD4+ T cell numbers occurs over years as the virus continuously infects and destroys these cells. AIDS is defined by a CD4 count falling below 200 cells per microliter or the occurrence of an AIDS-defining illness, and is characterized by profound immunosuppression leading to opportunistic infections and malignancies. HIV targets CD4+ T cells through binding of the viral envelope glycoprotein gp120 to the CD4 receptor and a coreceptor, either CCR5 (used primarily in early infection) or CXCR4 (which may emerge later), facilitating viral entry and integration into the host genome. The pathologic manifestations of HIV infection reflect both the direct effects of viral replication and the consequences of progressive immunodeficiency. Lymph nodes undergo an evolution from follicular hyperplasia in early infection, reflecting vigorous but ultimately futile immune activation, to follicular involution and eventual lymphoid depletion as the disease progresses and CD4+ T cells are destroyed. The brain is a major target, with HIV encephalitis producing multinucleated giant cells formed by the fusion of infected macrophages and microglia, contributing to the cognitive decline of HIV-associated neurocognitive disorder. The hallmark of advanced HIV disease is susceptibility to opportunistic infections that rarely affect immunocompetent individuals: Pneumocystis jirovecii pneumonia (PCP) with its characteristic bilateral interstitial infiltrates and foamy alveolar exudates, cytomegalovirus retinitis and colitis, Mycobacterium avium complex (MAC) disseminated infection, and mucocutaneous candidiasis. Influenza virus, an orthomyxovirus, causes respiratory infection ranging from mild upper respiratory illness to severe pneumonia and death, with pandemic potential arising from its capacity for antigenic variation. The virus targets respiratory epithelium, producing a necrotizing tracheobronchitis and bronchiolitis that strips the protective mucosal barrier. Complications include primary viral pneumonia, which may progress to acute respiratory distress syndrome, and bacterial superinfection, particularly with Staphylococcus aureus and Streptococcus pneumoniae, which colonize the denuded respiratory epithelium. The pathology of severe influenza pneumonia includes diffuse alveolar damage with hyaline membranes. Antigenic shift, the reassortment of genome segments between human and animal influenza strains, produces novel viruses to which the population has no pre-existing immunity, causing the periodic pandemics that have killed millions throughout history. COVID-19, caused by the novel coronavirus SARS-CoV-2, produces a spectrum of illness from asymptomatic infection to fatal multiorgan failure, with the respiratory system bearing the primary burden of severe disease. The virus enters cells by binding its spike protein to the angiotensin-converting enzyme 2 (ACE2) receptor, which is expressed on type II pneumocytes, endothelial cells, and numerous other cell types throughout the body. The pulmonary pathology of severe COVID-19 is characterized by diffuse alveolar damage (DAD) representing acute respiratory distress syndrome (ARDS), with hyaline membrane formation, type II pneumocyte hyperplasia, and notably prominent microthrombi in pulmonary vessels reflecting a coagulopathy. A hyperinflammatory state termed cytokine storm, driven by excessive release of pro-inflammatory cytokines including IL-6, TNF-alpha, and IL-1, amplifies tissue damage beyond that caused by the virus itself. Multi-organ involvement extends beyond the lungs to include cardiac injury with myocarditis, acute kidney injury, neurologic manifestations, and a thrombotic tendency that distinguishes COVID-19 from other viral pneumonias. Panel A: HIV pathogenesis showing viral entry via CD4/CCR5 receptors, progressive CD4 T cell depletion graphed over time, and lymph node changes from follicular hyperplasia to eventual depletion. Panel B: Influenza pneumonia with tracheobronchitis, necrotizing bronchiolitis, diffuse alveolar damage, and bacterial superinfection with neutrophilic infiltrate. Panel C: COVID-19 lung pathology demonstrating diffuse alveolar damage with hyaline membranes, type II pneumocyte hyperplasia, and microthrombi in pulmonary vessels. Panel D: HIV-associated opportunistic infections showing PCP with foamy alveolar exudate, CMV with owl-eye inclusions, and Kaposi sarcoma spindle cells. --- ### VIII. Fungal Infections Fungal infections are classified into four major categories based on the depth of tissue involvement and the immune status of the host. Superficial mycoses, caused by dermatophytes, are confined to the keratinized layers of skin, hair, and nails, producing the common conditions of tinea (ringworm). Subcutaneous mycoses, exemplified by sporotrichosis, involve the deeper skin and subcutaneous tissues following traumatic inoculation of organisms from the environment. Systemic endemic mycoses, caused by Histoplasma capsulatum, Coccidioides immitis, and Blastomyces dermatitidis, are dimorphic fungi capable of infecting immunocompetent individuals, producing disease that ranges from mild pneumonia to disseminated infection. Opportunistic fungi, including Candida, Aspergillus, Cryptococcus, and Mucor/Rhizopus, primarily infect immunocompromised hosts and have become increasingly important as medical advances create growing populations of susceptible patients. The systemic endemic fungi share the property of dimorphism, existing as molds in the environment and converting to yeast forms at body temperature, and are geographically restricted to specific regions. Histoplasma capsulatum is found in the Ohio and Mississippi River valleys, where it grows in soil enriched with bird and bat droppings, and its pathologic hallmark is the finding of small oval yeast forms within the cytoplasm of macrophages, reflecting its intracellular survival strategy. Coccidioides immitis inhabits the arid soils of the American Southwest, producing arthroconidia that are inhaled and convert to characteristic large spherules filled with endospores within infected tissues. Blastomyces dermatitidis predominates in the Southeast and Great Lakes regions, producing a distinctive large yeast with broad-based budding that is pathognomonic on histologic examination. Opportunistic fungi cause life-threatening infections in patients with specific immunologic defects. Candida species are the most common fungal pathogens, causing infections ranging from superficial mucocutaneous disease to disseminated candidiasis in immunocompromised and diabetic patients, with pseudohyphae representing their characteristic morphologic form. Aspergillus species, particularly A. fumigatus, are the most important cause of invasive mold infections in neutropenic patients, producing septate hyphae with acute-angle (dichotomous) branching that have a striking tendency for angioinvasion, causing thrombosis and tissue infarction. Cryptococcus neoformans is an encapsulated yeast that particularly targets patients with advanced AIDS when CD4 counts fall below 100, causing meningoencephalitis that is diagnosed by India ink preparation demonstrating the thick polysaccharide capsule surrounding the yeast cells. Mucor and Rhizopus species cause mucormycosis, a rapidly progressive and often fatal infection seen in patients with diabetic ketoacidosis (DKA) and iron overload, producing wide, ribbon-like, non-septate hyphae that invade blood vessels with the same destructive angioinvasion seen with Aspergillus. The tissue response to fungal infection varies considerably depending on the organism and the host immune status. Candida infections typically elicit a suppurative response with abundant neutrophils, reflecting the importance of neutrophil-mediated killing in defense against this organism. Histoplasma and Coccidioides provoke granulomatous inflammation that may be histologically indistinguishable from tuberculosis, requiring special stains such as GMS (Grocott methenamine silver) or PAS (periodic acid-Schiff) for definitive fungal identification. Aspergillus and Mucor produce angioinvasive infections with hemorrhagic infarction as the predominant pathologic finding, as hyphal invasion of vessel walls causes thrombosis and downstream tissue necrosis. Cryptococcus characteristically elicits a minimal inflammatory response because its thick polysaccharide capsule masks fungal antigens from immune recognition, producing a gelatinous exudate in the meninges without the granulomatous or suppurative inflammation that characterizes other fungal infections. Panel A: Histoplasma capsulatum showing small oval yeast forms within macrophage cytoplasm on GMS stain, granulomatous inflammation, and geographic distribution in Ohio and Mississippi River valleys. Panel B: Aspergillus with acute-angle branching septate hyphae on GMS stain, angioinvasion with vessel wall penetration, and characteristic pulmonary nodule with halo sign on CT. Panel C: Cryptococcus neoformans demonstrating thick polysaccharide capsule on India ink preparation, mucicarmine-positive yeast forms, and meningitis in AIDS patients with CD4 less than 100. Panel D: Mucormycosis with wide ribbon-like non-septate hyphae branching at right angles, angioinvasion causing tissue necrosis, and association with diabetic ketoacidosis and iron overload. --- ### IX. Parasitic Infections Protozoan parasites cause a diverse array of diseases that collectively affect billions of people worldwide, with clinical manifestations determined by the parasite's life cycle, tissue tropism, and the host immune response. Plasmodium species cause malaria, the most important parasitic disease globally in terms of morbidity and mortality. Entamoeba histolytica produces amebic dysentery with flask-shaped colonic ulcers and may disseminate to the liver to form amebic abscesses. Giardia lamblia colonizes the duodenum and jejunum, causing a malabsorptive diarrhea without mucosal invasion. Toxoplasma gondii is an obligate intracellular parasite that causes encephalitis in immunocompromised patients and devastating congenital infection when acquired during pregnancy. Trypanosoma species cause African sleeping sickness (T. brucei, transmitted by tsetse fly) and Chagas disease (T. cruzi, transmitted by reduviid bug), the latter producing a distinctive dilated cardiomyopathy. Leishmania species, transmitted by sandflies, cause a spectrum from self-limited cutaneous ulcers to fatal visceral disease (kala-azar) depending on the species and host immune status. Malaria pathology is dominated by the intraerythrocytic stage of the Plasmodium life cycle, in which merozoites invade red blood cells and undergo replication, eventually rupturing the cells and releasing new merozoites along with cellular debris. This cyclical pattern of invasion, replication, and erythrocyte destruction produces the characteristic periodic fevers that correspond to the synchronous lysis of infected red blood cells. Plasmodium falciparum causes the most severe form of malaria, uniquely producing cytoadherence of infected erythrocytes to vascular endothelium through expression of PfEMP1 on the red cell surface, leading to microvascular obstruction that causes cerebral malaria, the most feared complication. The spleen enlarges dramatically (splenomegaly) as it filters parasitized red blood cells and mounts an immune response. Hemozoin, or malarial pigment, represents the insoluble crystalline product of hemoglobin digestion by the parasite, and its accumulation within macrophages of the spleen and liver produces a characteristic slate-gray discoloration of these organs. Helminths represent a diverse group of parasitic worms classified into three major categories based on their morphology. Nematodes, or roundworms, include Ascaris lumbricoides (the most common human helminth), hookworms that cause iron deficiency anemia through intestinal blood loss, and pinworm (Enterobius vermicularis) that causes perianal pruritus, particularly in children. Cestodes, or tapeworms, include Taenia species acquired from undercooked pork or beef, and Echinococcus granulosus, which produces hydatid cysts in the liver and lungs following accidental ingestion of eggs from dog feces. Trematodes, or flukes, include Schistosoma species, which are the most pathologically significant helminths due to the chronic granulomatous inflammation their eggs provoke. Schistosomiasis exemplifies how the host immune response to parasitic eggs, rather than the adult worms themselves, drives the major clinical manifestations and organ damage. Schistosoma mansoni and S. japonicum deposit eggs in the mesenteric venous plexus, with eggs carried by portal blood flow to the liver where they lodge in periportal venules and provoke granulomatous inflammation. Over years, this chronic granulomatous reaction produces the characteristic periportal pipestem fibrosis, known as Symmers fibrosis, leading to portal hypertension, splenomegaly, and esophageal varices. Schistosoma haematobium, in contrast, deposits eggs in the vesical venous plexus draining the urinary bladder, producing granulomatous inflammation of the bladder wall that leads to hematuria, bladder calcification, and a significantly increased risk of squamous cell carcinoma of the bladder. The granulomas formed around schistosome eggs are composed of eosinophils, epithelioid macrophages, and giant cells surrounding the egg, which can be identified by its characteristic lateral spine (S. mansoni) or terminal spine (S. haematobium). Panel A: Malaria life cycle showing mosquito transmission, hepatocyte invasion, RBC infection with ring forms, trophozoites, and schizonts, and hemozoin pigment in spleen macrophages. Panel B: Entamoeba histolytica causing flask-shaped ulcers in colonic mucosa with narrow neck and broad base, trophozoites with ingested RBCs, and liver abscess with anchovy paste contents. Panel C: Schistosoma egg granuloma showing characteristic lateral (S. mansoni) or terminal (S. haematobium) spine on egg surrounded by eosinophils, epithelioid cells, and eventual fibrosis. Panel D: Schistosomiasis organ involvement comparing hepatic periportal pipestem fibrosis from S. mansoni/japonicum with bladder wall calcification and squamous cell carcinoma risk from S. haematobium. --- ### X. Host Defense and Special Topics Innate immunity provides the first line of defense against infectious agents through mechanisms that are immediately available and do not require prior exposure to a specific pathogen. Physical barriers, including intact skin and mucosal surfaces with their secreted antimicrobial peptides, mucus, and ciliary clearance, prevent the vast majority of potential pathogens from gaining access to internal tissues. Phagocytes, comprising neutrophils and macrophages, rapidly engulf and destroy pathogens through oxygen-dependent killing mechanisms including the respiratory burst and oxygen-independent mechanisms such as lysozyme and defensins. The complement system provides opsonization through C3b deposition, direct bacterial lysis via the membrane attack complex, and chemotactic recruitment of inflammatory cells through anaphylatoxins C3a and C5a. Interferons, particularly type I interferons alpha and beta, establish an antiviral state in neighboring uninfected cells by inducing expression of proteins that inhibit viral replication. Natural killer (NK) cells detect and kill virus-infected cells and tumor cells that have downregulated MHC class I expression, providing innate immune surveillance without requiring prior sensitization. Adaptive immunity provides highly specific and long-lasting protection against pathogens through the coordinated activities of B and T lymphocytes. Humoral immunity, mediated by antibodies produced by B cells and their plasma cell progeny, is most effective against extracellular pathogens and toxins, providing neutralization, opsonization, and complement activation. Cell-mediated immunity, carried out by T lymphocytes, is essential for defense against intracellular pathogens that are inaccessible to antibodies. CD4+ helper T cells coordinate the overall adaptive response by producing cytokines that direct the activities of other immune cells, with Th1 responses driving macrophage activation for intracellular pathogen control and Th2 responses promoting antibody production for extracellular pathogen defense. CD8+ cytotoxic T cells directly kill infected cells by recognizing pathogen-derived peptides presented on MHC class I molecules, delivering lethal hits through perforin and granzyme that induce apoptosis of the target cell. Specific immune defects predispose to characteristic patterns of infection that can guide clinical evaluation and diagnosis. Neutropenia, whether from chemotherapy, bone marrow failure, or other causes, produces profound susceptibility to bacterial and fungal infections because neutrophils are essential for rapid pathogen killing. T cell defects, whether from HIV infection, immunosuppressive medications, or primary immunodeficiency, predispose to infections with intracellular pathogens including mycobacteria, fungi, and viruses that require cell-mediated immunity for control. B cell and antibody deficiency leads to recurrent infections with encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis, which require opsonizing antibodies for efficient phagocytic clearance. Complement deficiency, particularly of terminal complement components C5-C9, produces specific susceptibility to Neisseria species, which are uniquely killed by the membrane attack complex. Splenectomy or functional asplenia removes a critical site of antibody production and phagocytic filtration, creating dangerous vulnerability to encapsulated organisms, particularly pneumococcus, mandating vaccination and often prophylactic antibiotics. Healthcare-associated infections represent an increasingly important category of infectious disease driven by the growing complexity of medical care, invasive devices, and antibiotic-resistant organisms. Catheter-related bloodstream infections are most commonly caused by Staphylococcus epidermidis, which forms biofilms on catheter surfaces, and Candida species, which colonize intravenous lines particularly in patients receiving total parenteral nutrition. Ventilator-associated pneumonia develops in mechanically ventilated patients when oropharyngeal secretions containing hospital-acquired organisms such as Pseudomonas aeruginosa and Acinetobacter baumannii are aspirated past the endotracheal tube cuff. Surgical site infections are most frequently caused by Staphylococcus aureus, which colonizes the skin and may contaminate the operative wound. Clostridioides difficile infection represents the prototypical antibiotic-associated infection, arising when broad-spectrum antibiotics disrupt the normal colonic flora and allow C. difficile to proliferate and produce its toxins, causing pseudomembranous colitis with potentially fatal complications. Panel A: Innate immunity components showing physical barriers (skin, mucosa), phagocytes (neutrophils, macrophages), complement cascade, interferons, and NK cells with their respective functions. Panel B: Adaptive immunity diagram comparing humoral response (B cells, antibodies targeting extracellular pathogens) with cell-mediated response (CD4 helper and CD8 cytotoxic T cells targeting intracellular pathogens). Panel C: Immunodeficiency susceptibility patterns showing neutropenia predisposing to bacteria and fungi, T cell defects to intracellular organisms, antibody defects to encapsulated bacteria, and complement defects to Neisseria. Panel D: Healthcare-associated infections showing catheter-related bloodstream infection with biofilm, ventilator-associated pneumonia, surgical site infection, and C. difficile colitis with pseudomembranes. --- ## Summary - Pathogens: bacteria, viruses, fungi, parasites, prions - Tissue responses: suppurative (bacteria), granulomatous (TB, fungi), cytopathic (viruses) - Gram-positive: S. aureus (abscess), Strep (cellulitis, pharyngitis), Clostridium - Gram-negative: E. coli pathotypes, Salmonella, Pseudomonas, Neisseria - TB: caseating granulomas, Ghon complex, reactivation in upper lobes - Herpes viruses: latency and reactivation; Cowdry A inclusions - HIV: CD4 depletion; opportunistic infections when CD4 <200 - Fungi: endemic (Histo, Cocci), opportunistic (Candida, Aspergillus) - Parasites: malaria (RBC invasion), schistosomiasis (granulomas) - Host defense: innate + adaptive; defects lead to specific susceptibilities --- ## Key Terms | Term | Definition | |------|------------| | Virulence factor | Pathogen trait contributing to disease | | Suppurative | Pus-forming inflammation | | Ghon complex | Primary TB lesion + draining lymph node | | Caseating necrosis | Cheesy necrosis in TB granulomas | | Cowdry A inclusion | Intranuclear inclusion in herpes infection | | Koilocyte | HPV-infected cell with perinuclear halo | | Opportunistic infection | Infection in immunocompromised host | | Angioinvasive | Fungal invasion of blood vessels | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*