A zoonotic disease refers to a pathogen that has been living within an animal, and then, for an environmental or genetic reason, jumps into the human population where it can cause disease.
Deeper dive
Two of the best-known zoonotic diseases are influenza and the plague. The flu virus lives in the guts of waterfowl. The flu can spread to humans through a genetic shift that causes people to become ill. The plague is caused by the bacteria, Yersinia pestis. It can live inside fleas, which then bite humans and cause illness. In 1346, rats carrying fleas with Yersinia pestis, traveled through trade routes in western Europe, causing a pandemic known as the Black Death. Around 60 percent to 75 percent of all new diseases that affect humans are zoonotic in origin.
Most scientists have concluded that COVID-19 is zoonotic and traced its origins to a wet animal market in Wuhan, China, though researchers have been unable to determine in which animal the reservoir for SARS-CoV-2 occurred. But, some researchers disagree with this assessment and theorize that the virus emerged from an accidental release at a Wuhan biology laboratory. In. June 2022, a World Health Organization science advisory group expressed the need for further investigation of the lab leak scenario.