Viral Zoonosis and Emerging Viral Pathogens

A viral zoonosis is an infection that is transmitted from animals to humans, and many of the viruses that newly emerge or re-emerge in human populations originate this way. This topic covers how viruses cross the species barrier from wildlife and livestock reservoirs, the factors that drive emergence, and notable examples including the filoviruses (Ebola, Marburg), bat-derived coronaviruses, Nipah and Hendra viruses, and zoonotic influenza.

Definition

Viral zoonoses are viral infections naturally transmitted between vertebrate animals and humans; emerging viral pathogens are viruses newly appearing in human populations or rapidly expanding in incidence or geographic range, a large proportion of which arise through zoonotic spillover from animal reservoirs.

Scope

The entry frames viral zoonoses and emerging viral pathogens as a category defined by cross-species transmission and the drivers of emergence, illustrated by representative agents. It is a reference overview of transmission ecology, virology, and epidemiology and does not provide clinical management or treatment guidance.

Core questions

• What ecological and viral factors allow a virus to cross from an animal reservoir into humans?
• Why do bats and certain other wildlife serve as reservoirs for so many emerging viruses?
• How do surveillance and the One Health approach help anticipate viral emergence?

Key concepts

• Zoonotic spillover
• Animal reservoir hosts
• Cross-species transmission and host adaptation
• Filoviridae (Ebola, Marburg)
• Bat-borne viruses
• Nipah and Hendra viruses
• Zoonotic and pandemic influenza
• One Health surveillance

Key theories

Wildlife origin of emerging infectious diseases

Analysis of emerging-disease events shows that a majority arise from wildlife and that their frequency has increased over recent decades, identifying zoonotic spillover as the dominant source of new human pathogens.

Mechanisms

Spillover occurs when a virus from an animal reservoir contacts and infects a human host, sometimes amplified by intermediate or amplifying hosts. Whether sustained human transmission follows depends on the virus's ability to use human receptors, replicate efficiently, and transmit between people, traits that can be acquired through mutation or reassortment. Ecological pressures such as land-use change, wildlife trade, and intensified animal contact increase the frequency of spillover events, and bats in particular harbour many viruses, including the ancestors of pathogenic coronaviruses.

Clinical relevance

Viral zoonoses include some of the most severe and feared human infections, such as Ebola and Marburg haemorrhagic fevers, as well as the bat-derived coronaviruses behind recent epidemics and a pandemic. Recognising the zoonotic origin of emerging viruses underpins outbreak investigation, surveillance, and prevention strategies at the human-animal interface. This entry is descriptive and not a basis for individual diagnosis or treatment.

Epidemiology

A majority of emerging infectious disease events in recent decades have originated in wildlife, and their frequency has risen over time; outbreaks of filoviruses, zoonotic coronaviruses, Nipah virus, and avian-origin influenza illustrate the recurrent threat posed by viral spillover.

Evidence & guidelines

Surveillance and ecological analyses quantify the wildlife origins and rising trend of emerging infections, and outbreak investigations of Ebola and bat-associated coronaviruses document the pathways of viral emergence; these inform the One Health framework for anticipating and responding to zoonotic threats (described at the level of evidence, not individual advice).

History

The recognition that most new human infections originate in animals consolidated through the late twentieth and early twenty-first centuries, as filovirus outbreaks, the 2003 SARS epidemic, Nipah virus emergence, avian and pandemic influenza, and the COVID-19 pandemic repeatedly traced back to wildlife reservoirs, establishing zoonotic spillover as the central concern of emerging-disease virology.

Key figures

• Peter Daszak
• Zheng-Li Shi
• Heinz Feldmann
• Kate Jones

Seminal works

• jones-2008
• cui-2018
• feldmann-2011-ebola

Frequently asked questions

What does it mean for a virus to be zoonotic?

A zoonotic virus is one that naturally transmits between animals and humans. Many emerging human viruses originate in animal reservoirs and spill over into people before, in some cases, spreading from person to person.

Why are bats so often linked to emerging viruses?

Bats are diverse, abundant, and long-lived, and they harbour a large range of viruses, including relatives of pathogenic coronaviruses and filoviruses, which can spill over to humans directly or through intermediate hosts.