Bats have a pretty spectacular immune system. They are practically disease free and live relatively long lives. When they get infections, they fight them off very well. In a study of two bat species, researchers found that both species were missing a gene segment called the cytokine storm.
The cytokine storm triggers an extreme and sometimes fatal reaction to infections. In addition to killing the infection, it also damages the organism's own tissue. Instead of this storm, bats' immune systems depress the inflammation to help them heal. Scientists believe that if we can figure out how they do this, we can find out how to better fight our own viruses.
The same genes that give bats their great immune system also allow them to fly. Bats are the only mammals capable of sustained flight. This flight, depending on how long it is, can be exhausting though. The exertion produces toxic free radicals that the bat's immune system must fight off. As such, their genes have evolved quickly to repair DNA damage.