Topic
Emerging Diseases & Zoonoses
How and why viruses emerge and cross from animals to people: the drivers of emergence and spillover, the viral evolution that powers host-jumping, and the surveillance and outbreak systems that contain them, with overview articles for two large virus groups, the arboviruses and the hantaviruses.
Most viruses that have emerged in humans over the past century came from animals. Of roughly 1,400 recognised human pathogens, more than half are zoonotic, meaning naturally transmitted between animals and people, and viruses are heavily over-represented among those that are genuinely new. A small number of animal reservoirs, above all bats and rodents, and a handful of mostly RNA-virus families supply most of these agents, from rabies and the influenzas to the filoviruses, arenaviruses and the arthropod-borne viruses.
This topic sets out the principles that govern how such viruses appear and are contained. Emergence and spillover covers the ecological and social drivers that bring a reservoir virus into contact with people, within the One Health view of human, animal and environmental health as one system. Viral evolution is the engine beneath emergence, since the high mutation rates, recombination and reassortment of viruses let them adapt to a new host. Outbreak response is the surveillance, investigation and preparedness machinery that turns detection into control.
The arboviruses and hantaviruses are two especially large, cross-cutting virus groups, each spanning many species, and each is given its own overview article within this topic. They sit among the wider set of zoonotic and emerging viruses rather than defining it.
→ See Drivers of Emergence, Spillover and One Health for what “emerging” means, the drivers that push a virus across the species barrier, the stages from spillover to sustained human transmission, the reservoir and vector cycles, the family map of where new viruses come from, and the One Health approach.
→ See Viral Evolution for the sources of viral variation (mutation, recombination, reassortment), quasispecies and the error threshold, selection and genetic drift, the molecular clock and phylogenetics, and how evolution drives host switching, immune escape and drug resistance.
→ See Outbreaks, Surveillance and Pandemic Preparedness for the steps of an outbreak investigation, the epidemic curve, surveillance systems and notifiable conditions, and the international framework for pandemic preparedness.
→ See Arboviruses for the ecological definition of an arbovirus, the sylvatic, epizootic and urban transmission cycles, the major virus families and their vectors, the four overlapping clinical syndromes, the tick-borne encephalitis complex, and diagnosis, vaccines and vector control.
→ See Hantaviruses for the rodent-borne hantaviruses, the New World cardiopulmonary and Old World renal syndromes, the immune-mediated pathogenesis of vascular leak, the Andes virus person-to-person exception, and their diagnosis and prevention.
Where emerging viruses come from
Most emerging human viruses arise from a few animal reservoirs and a small set of mostly RNA-virus families.
| Principal reservoir | Virus families | Notable human viruses |
|---|---|---|
| Bats | Coronaviridae, Filoviridae, henipaviruses, lyssaviruses | SARS-CoV-2, Ebola, Marburg, Nipah, Hendra, rabies |
| Rodents | Hantaviridae, Arenaviridae | Hantaviruses, Lassa virus |
| Wild birds (with pigs as a mixing host) | Orthomyxoviridae | Avian influenzas, pandemic influenza |
| Non-human primates | Retroviridae | HIV-1, HIV-2 |
| Vertebrate and arthropod cycles | Flaviviridae, Togaviridae, Bunyavirales | Dengue, yellow fever, Zika, West Nile, chikungunya, Rift Valley fever, Crimean-Congo haemorrhagic fever |
Key terms
| Term | Meaning |
|---|---|
| Zoonosis | An infection naturally transmitted between vertebrate animals and people. |
| Spillover | Transmission of a virus from its animal reservoir into a new host, including humans. |
| Reservoir host | The animal population that maintains a virus long term, usually without severe disease. |
| Amplifying host | A host in which the virus multiplies to high levels and bridges infection to people. |
| Dead-end host | An infected host that does not pass the virus on, so transmission stops. |
| One Health | The approach that treats human, animal and environmental health as one interconnected system. |
| Basic reproduction number (R0) | The average number of new infections one case produces in a fully susceptible population. |
| Quasispecies | The diverse swarm of related genomes within a host, on which selection acts as a whole. |
| Antigenic drift and shift | Gradual immune escape by point mutation (drift) versus abrupt change by reassortment (shift). |
| Molecular clock | The roughly constant rate of substitution that lets outbreaks be dated and traced. |
| Surveillance | The ongoing collection, analysis and dissemination of health data, tied to public-health action. |
| Notifiable condition | A disease that clinicians and laboratories are legally required to report to public-health authorities. |
| Public Health Emergency of International Concern | An extraordinary event, declared by the WHO, that poses cross-border risk and may need a coordinated response. |
| Quarantine versus isolation | Quarantine separates the possibly-incubating but well; isolation separates the already-infectious. |
References and recommended reading
- Burrell CJ, Howard CR, Murphy FA. Emerging Virus Diseases. In: Fenner and White’s Medical Virology, 5th edition, Chapter 15. Academic Press / Elsevier; 2017. The foundational account of emergence, spillover and zoonotic origins.
- Geoghegan JL, Holmes EC. Virus Evolution. In: Fields Virology, 7th edition (Fundamentals), Chapter 1. Wolters Kluwer; 2023. The current reference for the processes of viral evolution that underlie emergence.
- World Health Organization. International Health Regulations (2005), 3rd edition. WHO; 2016. The legal framework for international detection, reporting and response, including the Public Health Emergency of International Concern.
- World Health Organization. One Health. WHO fact sheet; 2026. The current reference for the One Health approach and the Quadripartite partnership.