Why do bats tolerate viruses that devastate other mammals?

Bats carry an unusual number of viruses that are lethal in other mammals - rabies, Marburg, and the coronaviruses closest to SARS and its relatives - and are largely untroubled by them. The leading explanation is that their immune systems run permanently part-activated, with an interferon response switched on all the time rather than triggered by infection, so a virus never gets an unopposed start.

That appears to be a side effect of flight. Flying is metabolically ferocious - a bat's resting rate is comparable to a running rodent's, and its body temperature rises to something a mammal would call a fever every time it goes out. Damaged DNA fragments from that effort would normally provoke inflammation, and bats seem to have dampened the inflammatory pathway while keeping the antiviral one, which is a trade almost nothing else has made.

The consequence for us is uncomfortable. Bats are not unusually dirty; they are unusually good hosts, and a virus that evolves to get past a permanently-armed immune system is a virus well equipped for anything it meets next. It also cuts the other way: understanding how a small mammal suppresses inflammation while living thirty years and rarely getting cancer is now a serious line of research in human medicine.