Drivers of Emergence, Spillover and One Health — Questions

A complete answer defines the term and names two distinct mechanisms.

Definition. An emerging viral infection is one newly recognised in a host or population, or a known infection expanding into a new geographical area or host species, often with a change in pathogenicity.

Mechanisms (any two):

• Zoonotic spillover from an animal reservoir, the dominant route, which introduces genuinely new genetic material into the human population.
• Genetic change in an existing virus: reassortment (as in influenza A), recombination, or mutation and selection that extends host range or virulence.
• Ecological or behavioural change that brings an endemic virus into new contact with people, or a breakdown of control (falling vaccine coverage) that allows re-emergence.

Human behaviour and movement repeatedly turn a contained reservoir virus into a human problem.

• Travel. Air travel disperses a virus globally within days, as the spread of SARS-CoV-2 from China to dozens of countries showed; the journeys an index case makes can decide whether an outbreak is containable.
• Trade and food systems. The wildlife trade and live-animal markets place reservoir species in contact with people, and intensive farming creates amplifying hosts.
• Encroachment. Deforestation and movement into wild habitat expose people to novel viruses and remove the predators that limit reservoir populations.
• Behaviour. Changes in sexual behaviour, injecting drug use and new medical procedures alter the transmission of endemic viruses such as the human immunodeficiency virus (HIV) and hepatitis B.
• Displacement and conflict. War and the collapse of public-health programmes allow re-emergence, as poliovirus did in Gaza in 2024.

Zoonotic spillover is the transmission of a virus from its animal reservoir into people. Most emerging viruses are zoonotic, and spillover is the event that introduces new viral genetic material into the human population.

For a spillover to lead anywhere, several barriers must be crossed: the virus must reach a susceptible person, attach to and enter human cells (usually requiring receptor compatibility), then replicate and shed efficiently enough to pass to the next host. Many zoonotic viruses cannot transmit between people, so their human basic reproduction number (R0) is near zero and every case needs fresh exposure to the reservoir. Spillover is more likely between closely related host species, which share similar cell receptors and habitat.

Roughly one third of emerging and re-emerging infections are caused by ribonucleic acid (RNA) viruses, out of proportion to their share of known viruses. The reason is their capacity for rapid evolution.

• High mutation rates. The RNA-dependent RNA polymerase lacks proofreading, generating about one mutation per genome per replication and a diverse variant population from which host-adapted mutants are selected.
• Large populations and short generation times mean advantageous variants arise and are selected quickly.
• Recombination and, in segmented genomes, reassortment produce sudden, large genetic change, including new host-range or virulence determinants.

Together these let an RNA virus adapt to a new host over days to weeks, against the years to millennia over which the host evolves its defences.

A complete answer links why displaced populations are at higher viral risk to the public-health response that risk demands.

Why displaced populations are at higher risk

• Overcrowding and poor sanitation in camps and informal settlements favour faecal-oral and respiratory spread (measles, hepatitis A and E, poliovirus).
• Collapsed services and vaccination gaps leave susceptible cohorts, allowing re-emergence of vaccine-preventable viruses, as with measles outbreaks and the return of poliovirus in conflict zones such as Gaza in 2024.
• Movement across boundaries carries viruses into new areas and exposes migrants to agents to which they have no immunity.
• Interrupted treatment for chronic viral infection (HIV, hepatitis B) worsens both individual outcomes and onward transmission.

The public-health response

• Catch-up and mass vaccination (measles, polio) on arrival.
• Surveillance with adapted case definitions, using syndromic surveillance where laboratory capacity is limited.
• Water, sanitation and shelter to interrupt transmission.
• Continuity of care for chronic infections, and integration of migrant health into national systems rather than parallel provision.

The unifying theme is that displacement removes the social and programmatic barriers that normally hold viruses in check, so the response must rebuild those barriers quickly.

When the human R0 is near zero the virus cannot sustain human chains, so every case depends on fresh spillover from the reservoir, the pattern seen with Lassa fever in West Africa. Only once R0 approaches and passes one can sustained human transmission occur.

An epizootic is an outbreak in an animal population, the animal equivalent of an epidemic, whereas enzootic transmission is the steady background maintenance of a virus in its reservoir. An epizootic frequently precedes human cases, as with West Nile virus in birds.

The World Health Organization estimates that more than 60% of emerging infectious diseases reported globally come from animals, and the share is higher still among genuinely new agents. This zoonotic dominance is why emergence is framed as a problem of the human-animal-environment interface.

The lower options understate the animal contribution; ~90% overstates it.

A successful jump is more likely between closely related hosts, which share less-divergent cell receptors and often the same habitat, giving both compatibility and exposure. Receptor compatibility is the key molecular barrier to host range.

Fruit bats are the Nipah reservoir, but pigs acted as the amplifying host in Malaysia, multiplying the virus and bridging it to farmers. (In the Bangladesh outbreaks transmission was direct, via date-palm sap.) Horses are the amplifying host for the related Hendra virus.

Humans and horses develop too little viraemia to infect feeding mosquitoes, so they are dead-end hosts; birds are the amplifying reservoir and Culex mosquitoes the vector. A dead-end host is infected but does not pass the virus onward.

Measles probably evolved from rinderpest, a virus of cattle, once human populations grew large enough to sustain transmission without an animal reservoir, the birth of a crowd disease. Smallpox, by contrast, is most closely related to camelpox.

Although viruses mutate constantly, pure mutation in human-adapted strains is an uncommon route to a new disease; most newly emerging viruses draw their new genetic material from infection of non-human species, that is, from zoonosis.

The other routes occur but do not account for the bulk of emergence.

An El Nino-driven drought-then-rainfall sequence produced a boom in deer-mouse numbers, raising human exposure to Sin Nombre virus and causing the outbreak. It illustrates how climate cycles drive reservoir population dynamics and spillover.

Each HIV-1 group arose from a separate spillover of simian immunodeficiency virus (SIV) from chimpanzees or gorillas, after which human-to-human transmission established the pandemic. The multiple groups are the signature of repeated independent cross-species transfers.

The urban cycle is a human-to-human loop transmitted by Aedes aegypti, distinct from the sylvatic (jungle) cycle that maintains these viruses among forest animals and forest mosquitoes. Anopheles transmits malaria, and Ixodes is a tick.

The Quadripartite comprises the WHO, FAO, WOAH and UNEP, which together produced the One Health Joint Plan of Action. The World Trade Organization is not part of it.