Zoonotic disease

A zoonotic disease is an infectious disease whose pathogen can be transmitted between animals and humans. The WHO estimates that approximately 60% of all established human infectious diseases and 75% of emerging diseases are zoonotic — humans share more pathogens with the animal kingdom than with each other.

Zoonotic transmission can occur through multiple pathways: direct contact (handling infected animals, bites, scratches), indirect contact (touching contaminated environments — soil, bedding, water), vector-borne (insects or arthropods carrying the pathogen between hosts), and foodborne (consuming contaminated animal products). The transmission pathway materially shapes the public-health response. Hantaviruses spread via inhalation of aerosolized rodent excreta. Ebola spreads from wildlife to humans through direct contact with infected animals or bushmeat. SARS-CoV-2 spilled over from an animal reservoir into humans and then established sustained human-to-human transmission.

Not every zoonotic spillover results in a sustained human outbreak. Most spillovers infect one or a few people and then end. Sustained human-to-human transmission requires that the pathogen's basic reproduction number in the human host (R0) exceeds 1. The pandemic risk score of an emerging zoonosis depends on (1) frequency of spillover events, (2) severity of human disease, and (3) potential for human-to-human transmission.