In One Health, what does spillover risk refer to?

• A. Transmission of a pathogen from animals to humans across species.
• B. Transmission of pathogens only within humans.
• C. Environmental contamination with no host involvement.
• D. Transmission exclusively among plants.

Answer: (A)

Spillover risk is the likelihood that a pathogen that is circulating in animals moves into humans and establishes infection across species. In One Health, this cross-species jump is the critical moment where animal health, human health, and the environment intersect, and understanding it helps with prevention and early detection of zoonotic diseases.

This concept matters because many human diseases originate in animals, often due to factors like increased contact between humans and wildlife, ecological changes, or pathogen traits that enable infection of new hosts. High spillover risk signals conditions that could lead to transmission from animals to people, potentially sparking outbreaks. Examples include viruses that leap from wildlife reservoirs into humans, sometimes via intermediary hosts, which is why surveillance in animals and managing human-animal interfaces are key strategies.

The other options describe different scenarios that do not capture cross-species transmission into humans: transmission confined to humans, environmental contamination without a host, or transmission among plants.