He worked with mice that had a genetic mutation common in people with Crohn’s disease. Those rodents developed inflamed guts, but only if they were infected by a virus that knocked out part of their immune system, and were exposed to an inflammatory toxin, and had a normal set of gut bacteria. If any of these triggers was missing, the mice stayed healthy. It was the combination of genetic susceptibility, viral infection, immune problems, environmental toxin, and their microbiome that gave them IBD. This complexity helps to explain why the disease is so variable.
