That’s because we eat a ton of meat, and the vast majority of it comes from factory farms. In these huge industrialized facilities that supply more than 90 percent of meat globally — and around 99 percent of America’s meat — animals are tightly packed together and live under harsh and unsanitary conditions.

“When we overcrowd animals by the thousands, in cramped football-field-size sheds, to lie beak to beak or snout to snout, and there’s stress crippling their immune systems, and there’s ammonia from the decomposing waste burning their lungs, and there’s a lack of fresh air and sunlight — put all these factors together and you have a perfect-storm environment for the emergence and spread of disease,“ said Michael Greger, the author of Bird Flu: A Virus of Our Own Hatching.

To make matters worse, selection for specific genes in farmed animals (for desirable traits like large chicken breasts) has made these animals almost genetically identical. That means that a virus can easily spread from animal to animal without encountering any genetic variants that might stop it in its tracks. As it rips through a flock or herd, the virus can grow even more virulent.

Greger puts it bluntly: “If you actually want to create global pandemics, then build factory farms.”