What is a zoonosis, and how does a virus jump from animals to people?

A zoonosis is an infectious disease that jumps from a non-human animal to a person. The germ behind it, called a pathogen, can be a virus, a bacterium, a parasite or a fungus. It is a very common route into our bodies: researchers estimate that about 60 percent of known human infectious diseases, and up to 75 percent of the newly emerging ones, are zoonotic in origin.

The jump itself is called spillover. It happens when animals that carry a germ in large numbers, the reservoir, come into contact with a new host, in this case us. Most spillovers go nowhere. A person catches rabies or anthrax and the chain stops there, with no one else infected. A few go further: Ebola, SARS and some bird flu viruses have started short chains of person-to-person spread. On rare occasions a microbe adapts so fully that humans become a stable new reservoir, which is what happened with HIV and with SARS-CoV-2, the virus that causes COVID-19.

Contact is the key ingredient, and people create it. In Malaysia in 1999, intensive pig farming began inside the habitat of fruit bats that carried Nipah virus. The pigs picked up the infection and passed it to the farmers, and 105 people died. As forests are cleared for human use, wild animals and people come closer together, and each meeting is another chance for a germ to cross.