Worldwide, bats are responsible for spreading an unusually wide variety of diseases that infect other animals and humans. What makes these small mammals such efficient viral incubators? According to one new study, their immune system gets a metabolic boost whenever they fly.

Why Are Bats So Good at Spreading Disease?

Bats possess an extraordinary portfolio of scary viruses, including Ebola, Marburg, Nipah and Hendra. Researchers have puzzled over this trait, trying to figure out what makes bats more efficient viral reservoirs than other disease-spreading critters. For instance, bats host more viruses per species than rodents, which is impressive when you consider that there are twice as many rodent species in the world.

An article appearing in the forthcoming issue of the CDC journal, Emerging Infectious Diseases, suggests an answer to this conundrum: bats are the only mammals than can fly.

During flight, bats exhibit a 15–16-fold increase in their metabolic rate—in comparison with the 7-fold increase in the metabolic rates of rodents running to exhaustion or the 2-fold increase in the metabolic rates of most flying birds.

Why does that matter? An elevated metabolism increases body temperature—which, like a fever, shortens the duration of a disease and improves the chances of survival in most animals. Higher body temperatures are known to trigger a number of immune responses, including the increased production of antibodies.

So bats are, essentially, constantly running a high fever. The authors of the study speculate that this serves as "a potent selective factor for the reduced virulence to the natural hosts seen in the pool of emerging viruses recently discovered in bats." In other words, bats can contract a disease, but it won't likely kill them—which makes them efficient carriers.

Good news if you're a bat. Not so good if you're a human being.