“Why Do Human Beings Keep Getting Diseases from Bats?”, 2020-03-30 (; similar):
Humans get a surprising number of very infectious diseases from bats. We get SARS (including the recent COVID-19/SARS-CoV2), Ebola, rabies, and possibly mumps. These are all incredibly infectious, deadly diseases. This seems weird because human beings aren’t in particularly close contact with bats. They’re nocturnal, don’t have large city populations (for the most part), and humans don’t eat them that often. It should be harder for diseases to pass from them to us. They’re also not very similar to us genetically, so their diseases shouldn’t be able to leap to us so easily.
Part of the answer is that bats are very social creatures. When one bat gets a virus, they pretty quickly pass it onto the other bats in their colony. However, that’s also true of goats and cows, who don’t seem to pass on infectious diseases to us as often.
…Bat cells do not work on a “see something, say something” model. Instead, bat cells just continually “say something”. Instead of recognizing viruses and then producing interferon, they continually produce interferon alpha and seem to produce almost no interferon beta: all gas, no brakes. In other words, bat cells just continually assume they’re under attack and never stop fighting viruses, regardless of whether they’ve detected any. This is surprising. Interferon is a really powerful molecule, and continually producing it should have the same effect on a cell as continually putting a factory on red alert. It should make the cell run much worse, and cause a lot of collateral damage.
…So, how do bats live so long with a hyperactive immune system? Well, the answer seems to be that although their interferon is continually produced, their immune system is never allowed to go to the same extremes as human immune systems…There’s a couple ways in which they don’t go to extremes. For one, bats seem to lack Natural Killer (NK) cell receptors, which may mean they lack NK cells…For another, bat cells also lack a lot of the pathways to go into apoptosis (self-destruct mode)…So, bats just live with the infections instead.
…Why are bats like this? What made their immune system so weird? Well, it actually has to do with their flying. Bats are the only mammals that fly. Flying is a really energetic process and can raise bats’ internal body temperature up to 41℃ (106℉) for an extended period of time. That’s really hot. In humans, that would cause serious brain damage. In bats, it’s enough to damage DNA through the production of reactive oxygen species, as well as to release the DNA into the cytoplasm or bloodstream.
This meant obviously that bats had to be really good at regularly repairing their DNA, a tricky process that can lead to cancer. But it also meant that bats couldn’t rely on the classic immune system trick of recognizing foreign pieces of DNA. In other animals, those were likely strands of DNA from a virus or bacteria. In bats, those were likely just pieces of bat DNA that had been damaged and let loose in the wrong place.
Recognition couldn’t work in the same way. So bats’ immune systems decided to be always on, instead. Then, to avoid the problems with that, bats’ immune system also evolved to never reach the same levels of inflammation as other mammals. The end result was that bats were much more able to live with deadly viruses, neither ignoring nor overreacting to them.