Uh oh: "We've heard obesity can be 'spread' between friends when we copy each other's eating habits, but a new study in mice suggests obesity could actually be infectious. That's right, infectious. As in, something you can catch. 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."