Zoonotic viruses: From spillover to pandemic preparedness

A virus of animal origin becomes a public health problem not simply because it has crossed a species boundary, but because animal, environmental and human interfaces continue to sustain opportunities for human infection. Zoonotic viruses are naturally transmissible from non-human vertebrates to humans, directly, through contaminated environments, food or animal products, or through arthropod vectors [1]. For risk assessment, the practical question is whether reservoirs, bridge or amplifying hosts, vectors, contaminated environments or animal products remain active contributors to human exposure. Reverse transmission also matters when human-derived infections alter infection ecology in animals, establish persistent animal reservoirs, or create conditions for further adaptation and spillback [2]. This distinction separates animal-origin viruses that have become maintained mainly through human transmission from those that still reach people through repeated animal-associated introductions.