Anyone who's ever visited a research lab that studies mice knows how the animals stink. But the mice housed in rows of large plastic bubbles in Jeffrey Gordon's lab at the Washington University School of Medicine smell surprisingly pleasant. They've spent their entire lives in a sterile, protected environment, inhaling purified air. Because of their meticulous upbringing, they harbor none of the microbes that normally give mice their distinctive acrid odor.

But living free of the bacteria that colonize most animals has also had a profound effect on the mice's development. They have less fat than their microbe-ridden counterparts and have to eat 30 percent more food to maintain their weight. Their hearts are 20 percent smaller, and they have immature immune systems.