Studying the DNA of two distant bat species, the scientists discovered how genes dealing with the bats' immune system had undergone the most rapid change.

This may explain why they are relatively free of disease and live exceptionally long lives compared with other mammals of similar size, such as the rat, said Professor Lin-Fa Wang, an infectious disease expert at the Duke-NUS Graduate Medical School in Singapore who led the multi-centre study.

"We are not saying bats never get sick or never get infections. What we are saying is they handle infections a lot better," Wang said in a telephone interview.

What was missing from both species of bats was a gene segment known to trigger extreme, and potentially fatal, immune reactions to infections, called the cytokine storm.

Cytokine storms end up killing not only offending viruses in the body, but the host's own cells and tissues too.