Imagine a virus standing at a border crossing. On one side are animals. On the other are humans. Most of the time, the border is secure. Cows remain with cows. Bats keep their viruses. Birds carry microbes that never bother us. Nature has built invisible fences over millions of years. But every now and then, a virus finds a way across. That single leap from an animal to a human is called zoonosis (plural: zoonoses).
