In the study, mice engineered to have a particular immune deficiency developed fatty liver disease and got fatter when fed a Western-style diet. But strikingly, when these immune-deficient mice were put in the same cage as healthy mice, the healthy mice started to come down with symptoms of liver disease, and also got fatter.

The culprit? Microbes in the stomachs of the mice. Because the mice had their immune systems disturbed, the bacteria in their guts got “out of wack,” said study researcher Richard Flavell, a professor of immunobiology at Yale School of Medicine. We normally live in symbiosis with the bacteria in our guts, but in the study, the number of “bad,” disease-associated bacteria increased 1,000-fold in mice with immune problems, Flavell said.

And it’s these bad bacteria that were transmitted from mouse to mouse, causing the healthy mice to also experience changes in their gut microbes — and making them fat.