# Lecture 28: Emerging Infectious Diseases
## Microbiology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Define emerging and re-emerging infectious diseases and provide examples of each
2. Identify the major factors driving the emergence of new infectious diseases
3. Describe the concept of zoonotic spillover and the role of animal reservoirs
4. Discuss notable emerging infections of the 21st century including SARS, MERS, Ebola, Zika, and SARS-CoV-2
5. Explain the principles of pandemic preparedness and response
6. Discuss the role of genomic surveillance, One Health, and international coordination in addressing emerging threats
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## Lecture Content
### I. Definitions and Scope
An emerging infectious disease (EID) is defined as a disease that has newly appeared in a population or one that has existed previously but is rapidly increasing in incidence or geographic range. Examples include SARS-CoV-2, Ebola, Zika, MERS, H5N1 avian influenza, Nipah virus, and mpox. A re-emerging infectious disease is one that was previously declining but is now resurging; examples include tuberculosis (particularly MDR and XDR-TB), measles (driven by vaccine hesitancy), dengue (expanding its geographic range), yellow fever, and cholera.
Some diseases are classified as newly recognized, meaning they existed previously but were only recently identified. Examples include hepatitis C (identified in 1989), Helicobacter pylori gastritis (recognized in 1983), Lyme disease (described in 1975 and the organism identified in 1982), and hantavirus pulmonary syndrome (recognized in 1993).
Approximately 75% of emerging infectious diseases are zoonotic in origin, arising from animal-to-human transmission. The frequency and geographic scope of EID events have increased significantly over the past several decades, driven by the convergence of multiple ecological, behavioral, and microbiological factors.
### II. Factors Driving Emergence
#### A. Ecological and Environmental
Deforestation and land-use change destroy wildlife habitats and increase contact between humans, domestic animals, and wildlife reservoirs. The emergence of Nipah virus in Malaysia provides a compelling example: fruit bats displaced by deforestation moved to pig farms, the virus spilled over from bats to pigs, and then from pigs to humans. Climate change is expanding the geographic range of disease vectors such as mosquitoes and ticks. Dengue, chikungunya, and Zika are moving into previously temperate regions, while tick-borne diseases like Lyme disease and babesiosis are expanding northward. Rapid, unplanned urbanization creates ideal conditions for vector-borne disease transmission (standing water provides breeding sites for Aedes mosquitoes) and crowding facilitates respiratory pathogen transmission. Agricultural intensification, particularly factory farming, creates high-density animal populations where pathogens can amplify and undergo genetic reassortment, as demonstrated by the 2009 swine flu (H1N1), avian influenza H5N1, and H7N9.
#### B. Human Behavior and Demographics
Global travel and trade allow infected individuals to traverse continents within hours, as dramatically demonstrated when SARS spread to 29 countries within weeks in 2003 and when SARS-CoV-2 became a pandemic. Population growth and density, particularly in megacities and slums with inadequate sanitation, create conditions ripe for disease emergence. Armed conflict and displacement break down public health infrastructure and concentrate vulnerable populations in refugee camps with crowding and poor hygiene, fueling outbreaks of cholera and measles. Bushmeat hunting and wet markets bring humans into direct contact with wildlife, increasing the risk of zoonotic spillover, as occurred with HIV (from primates), Ebola (from bats), and SARS-CoV (from civets or raccoon dogs). Changes in sexual behavior and intravenous drug use drive the spread of HIV, hepatitis B and C, and mpox (clade IIb through sexual transmission networks). Antimicrobial misuse drives the emergence of multidrug-resistant pathogens, as discussed extensively in previous lectures.
#### C. Microbial Adaptation
Microorganisms are constantly evolving through mutation and genetic reassortment. Influenza undergoes antigenic shift through genome segment reassortment, while SARS-CoV-2 variants (Alpha, Delta, Omicron) accumulated mutations conferring increased transmissibility or immune evasion. Horizontal gene transfer enables the acquisition of virulence factors and resistance genes. Host switching occurs when a pathogen that normally infects animals acquires the ability to infect and transmit between humans, representing the critical step toward pandemic emergence. Biofilm formation, persistence mechanisms, and latency contribute to chronic and recurring infections that are difficult to eradicate.
#### D. Public Health Infrastructure
Vaccine hesitancy and declining vaccination coverage have driven the global resurgence of measles. Weakened public health systems, resulting from chronic underfunding of surveillance networks, laboratory capacity, and the public health workforce, impair the ability to detect and respond to novel threats. The deliberate release of biological agents (bioterrorism), as demonstrated by the 2001 anthrax letters, represents an additional threat that demands preparedness.
A cause-and-effect diagram of factors driving emerging infectious diseases. Central hub labeled "Emerging Infectious Diseases" with six radiating spokes, each leading to a category. Spoke 1 (Ecological): icons of deforestation, climate change (thermometer), urbanization (city skyline); examples -- Nipah (bat to pig), tick-borne disease range expansion. Spoke 2 (Human behavior): icons of airplane (global travel), bushmeat market, IV needle; examples -- SARS global spread, HIV zoonotic origin. Spoke 3 (Microbial evolution): icons of mutating virus, plasmid transfer; examples -- influenza antigenic shift, SARS-CoV-2 variants, AMR. Spoke 4 (Agriculture): icons of poultry farm, pig farm; examples -- avian influenza H5N1, swine flu H1N1. Spoke 5 (Demographics): icons of population density graph, refugee camp; examples -- cholera in displacement settings, urban dengue. Spoke 6 (Public health gaps): icons of broken vaccine syringe, defunded laboratory; examples -- measles resurgence, delayed outbreak detection. Each spoke includes 2-3 specific examples with dates.
### III. Zoonotic Spillover
Spillover is the transmission of a pathogen from an animal reservoir to a human host. The reservoir host is the animal species that maintains the pathogen without experiencing significant disease. Bats serve as extraordinary reservoir hosts for many high-consequence viruses, including Ebola, Marburg, Nipah, Hendra, SARS-like coronaviruses, and rabies. Several features of bat biology explain this role: their long lifespans, colonial roosting behavior (with millions congregating together), capacity for flight (providing wide geographic range), and a unique immune system characterized by tolerance of viral infection without disease through a dampened inflammatory response and constitutive interferon expression.
An intermediate or amplifying host is an animal that may amplify the pathogen and bring it into closer proximity to humans. Known intermediate hosts include civets and raccoon dogs for SARS-CoV, dromedary camels for MERS-CoV, pigs for Nipah virus, and domestic poultry for avian influenza.
Several factors determine whether spillover occurs: the viral load in the reservoir, the frequency and nature of human-animal contact, the host range of the virus (including receptor compatibility), and the level of population immunity in humans. Most spillover events are dead-end infections that do not lead to sustained human-to-human transmission. Achieving sustained transmission represents the critical step that confers pandemic potential. Viral evolution at the human-animal interface, including mutations that enhance binding to human receptors (such as SARS-CoV-2 spike protein affinity for ACE2), increase transmissibility, or enable immune evasion, determines whether this threshold is crossed.
### IV. Notable Emerging Infections of the 21st Century
#### A. SARS (2002-2003)
SARS-CoV emerged from a bat reservoir through a civet or raccoon dog intermediate host, originating in Guangdong, China. It caused severe respiratory disease, with approximately 8,100 cases and 774 deaths (a case fatality rate of about 10%). Super-spreader events in hospitals and hotels drove much of the transmission. The outbreak was contained through aggressive quarantine, contact tracing, and travel advisories, with no sustained community transmission occurring since 2004. The SARS experience demonstrated that a novel respiratory virus could spread globally via air travel and catalyzed significant investment in coronavirus research.
#### B. Influenza Pandemics and Pandemic Threats
The 2009 H1N1 pandemic was caused by a triple-reassortant virus containing human, swine, and avian gene segments that emerged in Mexico and rapidly spread worldwide. Although relatively mild in overall severity, it disproportionately affected young adults and highlighted the critical importance of pandemic preparedness planning. H5N1 avian influenza is highly pathogenic in poultry and causes sporadic human infections with a case fatality rate exceeding 50%. Although limited human-to-human transmission has occurred, concern persists about mutations that could enable sustained transmission. Outbreaks in dairy cattle during 2024-2025, with occasional human cases, have renewed vigilance regarding this threat. H7N9 emerged in China in 2013 from a poultry reservoir, causing over 1,500 cases with a case fatality rate of approximately 39%, and remains under ongoing surveillance.
#### C. Ebola Virus Disease
Ebola virus (family Filoviridae) is suspected to originate from a bat reservoir, with transmission occurring through direct contact with infected blood and body fluids. The West Africa outbreak of 2013 to 2016, affecting Guinea, Liberia, and Sierra Leone, was unprecedented in scale, with over 28,000 cases and more than 11,000 deaths. It overwhelmed healthcare systems, with traditional burial practices and nosocomial transmission driving spread. Multiple outbreaks in the Democratic Republic of the Congo, including the 2018-2020 North Kivu outbreak, have been complicated by ongoing armed conflict and community mistrust.
Clinically, Ebola presents with abrupt onset of fever, myalgia, and gastrointestinal symptoms (vomiting and diarrhea), with hemorrhagic manifestations appearing late in the course. The case fatality rate varies from 25 to 90% depending on the viral species and outbreak circumstances. Treatment now includes monoclonal antibodies (Inmazeb, comprising atoltivimab, maftivimab, and odesivimab) along with supportive care. Prevention relies on the rVSV-ZEBOV vaccine (Ervebo), which is highly effective and deployed through a ring vaccination strategy.
#### D. MERS (2012-present)
MERS-CoV originated in bats and uses the dromedary camel as an intermediate host, with cases concentrated on the Arabian Peninsula. Approximately 2,600 cases have been documented with about 900 deaths (a case fatality rate of roughly 35%). Human-to-human transmission is limited and predominantly nosocomial. Ongoing zoonotic spillover from camels continues to produce sporadic cases.
#### E. Zika Virus (2015-2016)
Zika virus is an Aedes mosquito-borne flavivirus that can also be transmitted sexually and vertically. The 2015-2016 outbreak centered in Brazil spread throughout the Americas, revealing the devastating consequence of congenital Zika syndrome, which causes severe microcephaly, ocular defects, and arthrogryposis. An association with Guillain-Barre syndrome was also established. The WHO declared a Public Health Emergency of International Concern (PHEIC) in 2016. No specific treatment or licensed vaccine is currently available.
#### F. SARS-CoV-2 / COVID-19 (2019-present)
SARS-CoV-2 emerged in Wuhan, China in late 2019, likely originating from bats with a debated intermediate host (pangolin, raccoon dog, or possibly direct spillover). Its spike protein binds the ACE2 receptor, and the virus is highly transmissible via respiratory droplets and aerosols. The clinical spectrum ranges from asymptomatic infection to acute respiratory distress syndrome, cytokine storm, and multi-organ failure, with long COVID (post-acute sequelae of SARS-CoV-2) affecting many survivors.
The pandemic's global impact has been staggering, with over 770 million confirmed cases and more than 7 million deaths (likely significant underestimates), accompanied by massive economic disruption. Variant evolution has followed a pattern of increasing transmissibility: Alpha and Delta (which also increased severity), then Omicron (which showed increased transmissibility and immune evasion but less severe disease), with ongoing sub-lineage evolution.
Countermeasures were developed at unprecedented speed. mRNA vaccines (BNT162b2 and mRNA-1273) and viral vector vaccines were produced in approximately 11 months. Antivirals include nirmatrelvir-ritonavir (Paxlovid), remdesivir, and molnupiravir. Monoclonal antibodies were initially effective but many lost efficacy against Omicron variants. Public health measures encompassed masking, physical distancing, testing, and contact tracing. Key lessons include the importance of rapid genomic surveillance, the challenge of achieving global vaccine equity (as attempted through COVAX), the need for sustained preparedness infrastructure, and the complex political and social dimensions of pandemic response.
#### G. Mpox (Monkeypox, 2022-present)
Monkeypox virus (an Orthopoxvirus) has a rodent reservoir in Africa and was historically endemic in Central and West Africa. The 2022 global outbreak (clade IIb) spread predominantly through sexual contact networks, presenting with vesiculopustular rash and lymphadenopathy. The 2023-2024 DRC outbreak (clade Ib) proved more severe, with both sexual and household transmission, prompting a PHEIC declaration. Vaccines (JYNNEOS/MVA-BN) and antivirals (tecovirimat/TPOXX) are available for prevention and treatment.
A timeline of major emerging infectious disease events from 2000 to 2025. Horizontal timeline with events marked by icons and brief descriptions. 2002-2003: SARS-CoV (bat icon, civet icon, respiratory illness; 8,100 cases). 2009: H1N1 pandemic influenza (pig icon; global spread). 2012: MERS-CoV (camel icon; Arabian Peninsula; CFR 35%). 2013-2016: Ebola West Africa (bat icon, contact transmission; 28,000+ cases; Ervebo vaccine developed). 2015-2016: Zika (mosquito icon; congenital Zika syndrome; microcephaly). 2019-present: SARS-CoV-2/COVID-19 (bat/unknown intermediate; respiratory/aerosol; 770M+ cases; mRNA vaccines in 11 months). 2022-present: Mpox global outbreak (rodent reservoir; sexual transmission networks; JYNNEOS vaccine). Each event includes an arrow indicating whether the outbreak was contained, became endemic, or is ongoing. A background shading shows the increasing frequency of EID events over time.
### V. Pandemic Preparedness and Response
#### A. Surveillance and Early Warning
Genomic surveillance through whole-genome sequencing of circulating pathogens enables detection of novel variants, tracking of transmission chains, and identification of resistance mutations. GISAID facilitates global sharing of influenza and SARS-CoV-2 genomic data. Wastewater-based epidemiology, which monitors sewage for pathogen genetic material, has emerged as a valuable early indicator of community transmission. Syndromic surveillance automates the monitoring of emergency department visits, pharmacy sales, and digital data (including search trends and social media) for early anomaly detection. Established alert systems such as ProMED, GPHIN, HealthMap, and WHO Disease Outbreak News provide early warning of unusual disease events.
The International Health Regulations (IHR) constitute the WHO framework requiring member states to develop core capacities for surveillance, laboratory testing, and response, including the obligation to notify WHO of Public Health Emergencies of International Concern (PHEICs).
#### B. Response Frameworks
Effective pandemic response requires organized coordination through the incident command system (ICS), a standardized management structure for emergency response, and Emergency Operations Centers (EOCs) that serve as coordination hubs. Contact tracing, whether conducted through digital applications or traditional methods, identifies and monitors individuals exposed to confirmed cases. Non-pharmaceutical interventions (NPIs) include isolation of cases, quarantine of contacts, social distancing, masking, hand hygiene, school and business closures, and travel restrictions.
Medical countermeasures encompass rapid development and deployment of diagnostics, therapeutics, and vaccines. The 100 Days Mission, championed by CEPI (Coalition for Epidemic Preparedness Innovations), aims to have diagnostics, therapeutics, and vaccine candidates ready within 100 days of identifying a new pandemic threat.
#### C. Vaccine Development in a Pandemic
Several platform technologies have dramatically accelerated vaccine development. mRNA platforms can design and produce vaccine candidates within days of obtaining a pathogen's genetic sequence. Viral vector platforms offer a plug-and-play approach with established safety profiles. Protein subunit platforms combined with established adjuvants provide another option. Regulatory pathways such as Emergency Use Authorization (EUA) in the United States and Emergency Use Listing (EUL) through WHO allow deployment of vaccines and therapeutics during emergencies based on available safety and efficacy data without waiting for full licensure. Clinical trial innovations including adaptive trial designs, platform trials (testing multiple candidates simultaneously), and human challenge studies further accelerate the timeline from sequence to vaccine.
### VI. One Health Approach
One Health is the recognition that human health, animal health, and environmental health are fundamentally interconnected, and that preventing the next pandemic requires collaboration across these traditionally separate disciplines. Human medicine contributes clinical care, diagnostics, and public health surveillance. Veterinary medicine provides surveillance of animal diseases, livestock biosecurity, and wildlife monitoring. Environmental science offers monitoring of ecosystem changes, deforestation impacts, water quality, and climate-health linkages.
Practical applications of the One Health approach include surveillance at the human-animal interface (in live animal markets and farms), monitoring of antimicrobial resistance across human, animal, and environmental settings, joint outbreak investigation teams, and reducing deforestation to minimize spillover risk. The Quadripartite alliance of WHO, FAO, WOAH (World Organisation for Animal Health), and UNEP coordinates One Health implementation at the global level.
### VII. Bioterrorism and Deliberate Release
The CDC classifies potential bioterrorism agents by threat level. Category A agents carry the highest priority because they are easily disseminated, carry high mortality, have the potential for public panic, and require special preparedness. These include Bacillus anthracis (anthrax), Yersinia pestis (plague), Clostridium botulinum toxin (botulism), Francisella tularensis (tularemia), Variola major (smallpox), and the filoviruses (Ebola and Marburg).
The 2001 anthrax letters, in which weaponized B. anthracis spores were mailed to media offices and U.S. senators, caused 22 cases (11 inhalational) and 5 deaths, starkly highlighting the nation's vulnerability and catalyzing major investment in biodefense. Preparedness infrastructure now includes the Strategic National Stockpile (SNS) of medical countermeasures, the Laboratory Response Network (LRN) for rapid pathogen identification, BioWatch for environmental monitoring, and BARDA (Biomedical Advanced Research and Development Authority) for medical countermeasure development.
Dual-use research of concern (DURC) refers to legitimate scientific research that could potentially be misused to pose a biological threat, such as gain-of-function research on influenza. This research is subject to oversight by institutional biosafety committees and governed by national policies that seek to balance scientific progress with biosecurity.
### VIII. Future Outlook
Emerging infectious diseases will continue to arise; the question is not whether a new threat will appear but when and what form it will take. The WHO concept of "Disease X" represents an as-yet-unknown pathogen that could cause a future pandemic, emphasizing that preparedness must include platform technologies and flexible response capabilities rather than focusing solely on known threats.
Key priorities for the future include strengthening global surveillance networks and genomic sequencing capacity, investing in broad-spectrum antivirals and universal vaccine platforms, addressing root causes by reducing deforestation, regulating the wildlife trade, improving sanitation, and closing the equity gap in healthcare access. Building resilient public health systems, maintaining public trust, and fostering international cooperation and data sharing must remain cornerstones of preparedness if humanity is to face the inevitable next pandemic from a position of strength rather than scrambling to catch up.
A conceptual framework for pandemic preparedness. Central triangle labeled "Pandemic Preparedness" with three sides representing: (1) Prevent -- reduce spillover risk through One Health interventions (wildlife trade regulation, deforestation reduction, farm biosecurity); icons of forest, livestock, and wild animals. (2) Detect -- early warning systems including genomic surveillance (DNA helix and sequencing machine), wastewater monitoring (sewage pipe with virus particles), syndromic surveillance (computer dashboard with alert), and IHR reporting (WHO logo). (3) Respond -- rapid deployment of countermeasures: diagnostics (PCR machine), therapeutics (antiviral pill, monoclonal antibody vial), vaccines (mRNA LNP schematic and syringe), and non-pharmaceutical interventions (mask, social distancing icons); the "100 Days Mission" target is displayed. Outside the triangle, surrounding arrows show enabling factors: international cooperation, equitable access, sustained funding, workforce training, community engagement, and research and development pipelines.