Emerging and Re-emerging Viruses

Viral emergence and re-emergence are driven by stable biological and epidemiological principles such as zoonotic spillover, viral adaptation, vector expansion, mutation, reassortment, travel, and population immunity.

An emerging virus is a virus newly identified in the human population, or newly recognised as a cause of disease. It may be genuinely novel, or have only recently evolved the capacity to infect and spread among humans. It may also be simply newly detected due to improved surveillance and diagnostic capability.

A re-emerging virus is a previously known, previously controlled or declining virus that increases again in incidence or geographic range. This reflects a change in some combination of viral, vector, host, or societal factors rather than the appearance of a genuinely new pathogen.

Zoonotic spillover is the single most important source of genuinely novel emerging human viral pathogens. A virus circulating in an animal reservoir crosses into the human population, either directly or via an intermediate amplifying host.

Direct crossing is bat-to-human, as suspected for the filoviruses. Intermediate hosts include the civet cat implicated in the original SARS-CoV emergence, and the pig mixing vessel role in some influenza reassortment events.

Spillover risk is directly shaped by the frequency and intimacy of human-animal contact. Agricultural intensification, wildlife trade, deforestation-driven encroachment into previously undisturbed wildlife habitat, and live animal markets are all recognised structural drivers. Spillover is not a purely random or unpredictable biological event.

A successful spillover event producing a genuinely novel human pathogen additionally requires the virus to achieve, either immediately or through subsequent adaptation, sufficient human-to-human transmissibility to sustain onward spread.

Many zoonotic spillover events, several of the individual case exposures to avian influenza strains, for example, produce severe human disease without ever achieving efficient sustained human-to-human transmission. They remain epidemiologically contained as isolated or limited-cluster events rather than progressing to a sustained outbreak or pandemic.

Following initial spillover, a virus may require further genetic adaptation, accumulated point mutation, to improve its efficiency of human-to-human transmission, human receptor binding, or replication within human tissue.

This process can occur over the course of an ongoing outbreak itself. Progressively more transmissible variants emerge and are selected for as the outbreak continues.

The well-documented emergence of successive SARS-CoV-2 variants of concern over the course of the COVID-19 pandemic is the clearest recent illustration of this principle in real time. Successive variants showed progressively altered transmissibility, immune evasion, or both, relative to earlier circulating lineages.

For arboviruses specifically, geographic range is fundamentally vector-determined. Any factor expanding a vector's own geographic or seasonal range directly expands the corresponding virus's potential range and transmission season.

Climate change is the dominant contemporary driver of this phenomenon. It has produced documented poleward and altitudinal expansion of the geographic range of several key mosquito vector species, notably Aedes species relevant to dengue, Zika, and chikungunya.

This brings previously unaffected regions into potential transmission range for the first time. It also extends the transmission season in regions already within range.

Urbanisation and increased international travel and trade compound this vector-driven expansion. They increase both the density of susceptible human populations in newly vector-suitable areas and the opportunity for infected travellers to seed local transmission where a competent vector is now present.

For segmented viruses specifically, influenza is the paradigm and, among human pathogens, the dominant example. Reassortment between co-infecting strains produces abrupt, large-scale antigenic change in a single step.

This is the mechanism underlying genuine pandemic emergence, as distinct from the more gradual, drift-driven seasonal epidemic pattern.

Every major historical influenza pandemic reflects a reassortment event producing a novel haemagglutinin and neuraminidase combination against which the human population had little or no pre-existing immunity. Reassortment surveillance in animal, particularly avian and swine, influenza populations is therefore a specific, actively maintained global public health priority.

The gradual, ongoing background process of mutation contributes to both emergence and re-emergence. In emergence, it takes the form of accumulated adaptive mutation following spillover.

In re-emergence, antigenic drift erodes previously effective population immunity or vaccine efficacy over time. This is most classically illustrated by the need for annual influenza vaccine reformulation, but is relevant in principle to any sufficiently mutable virus.

RNA viruses, given their characteristically higher mutation rate, are disproportionately represented among both emerging and re-emerging viral threats relative to the comparatively more genetically stable DNA viruses.

Modern international travel volume and speed mean a virus can move from an isolated initial outbreak location to multiple distant countries within the same incubation period as the disease itself. This fundamentally compresses the timeline available for containment compared to any previous era in human history.

This is of major relevance to why contemporary outbreak response emphasises rapid international reporting and coordinated response.

SARS-CoV-2 spread globally within weeks of initial recognition. SARS-CoV, two decades earlier, followed a more geographically contained spread pattern. The difference reflects both the specific virus's own transmission characteristics and, to a real degree, the further increase in global travel volume and connectivity in the intervening years. Together they illustrate this principle's real-world consequence.

Population-level susceptibility is the final determinant of whether an emerging or re-emerging virus achieves large-scale spread once introduced. It reflects the balance between naturally acquired immunity from prior infection, vaccine-induced immunity, and genuinely naive, previously unexposed, population segments.

Waning immunity, whether from natural infection or vaccination, is a recognised driver of re-emergence for several previously well-controlled diseases. It may reflect genuine decline in protective antibody or T cell responses over time, or viral antigenic change outpacing existing immunity.

Pertussis resurgence in some settings and periodic measles outbreaks in under-vaccinated population pockets both illustrate this principle. Measles vaccination is highly effective when uptake is adequately maintained, so this directly demonstrates that re-emergence often reflects a societal or vaccination-coverage failure rather than a change in the virus itself.

Immune naivety in a population never previously exposed to a given virus, or antigenic variant, is the corresponding driver for genuinely novel emergence. This connects directly back to the antigenic shift and pandemic potential concept.