How do bats survive their own coronaviruses (without showing any symptoms)?
Or, more generically, how can viruses keep reproducing inside healthy carriers without inducing any pathogenic effect?
Are coronaviruses able to replicate themselves without harming bats, or maybe these viruses are just latent like herpesviruses in humans?
Related question: Why are bats the source of dangerous coronavirus pandemics?
It's common for the reservoir host of a zoonotic virus to be tolerant of it. MERS coronavirus appears to cause mild or no disease in dromedary camels ( source ), but kills about 35% of confirmed infected humans. ( CDC ) Sin Nombre hantavirus seems to be mild in the deer mice that spread it, despite ~36% fatality rate in humans. ( source ) Mosquitoes are efficient vectors for flaviviruses like dengue and zika in part because they have adaptations we lack that protect them from the virus. ( source ) Also, human communities are host to several viruses: about 90% of people have a herpesvirus infection ( source ) with similar numbers for polyomaviruses. ( source ) Very few of these infected individuals show symptoms.
The wide prevalence of these asymptomatic infections shows that the virus is successful when it can replicate while the host remains healthy. In general, virus reproduction kills cells, and when cells die faster than the host can replace them, this causes symptoms up to and including death. The host's immune system supresses virus activity, and a virus that can't evolve ways to avoid immune supression will be wiped out. But if the virus gets so good at avoiding the immune system, it will kill the host, which is bad for the virus. So both host and virus tend to evolve to a point where the immune system wins most, but not all of the time. There is a lot more to it, but I'll stop here.
In conclusion, we don't really need to look for special properties of bats to explain their tolerance of coronaviruses, even though, as iayork points out, there are reasons to expect bats might be more resistant.
It's been tentatively proposed that bats are often unusually able to tolerate long-term infection with a wide range of viruses (though this hasn't been formally shown to be true). A specific cause for this (possible) phenomenon isn't known, but many possible explanations have been put forward. It's likely that if it's true, there's no single cause but rather a combination of multiple causes to add together.
Bats have an array of unique life history characteristics that not only allow them to be particularly good reservoirs for viruses that are highly pathogenic in other species, but also appear to have shaped their immune systems. Although research on bat antiviral immunity has focused on only a few species to date, at the genomic level, selection on genes is concentrated on the innate immune system across both suborders of bats. However, while these studies have provided a rich source of hypotheses, the majority remain to be tested at the functional level and many questions remain that cannot be answered from comparative genome studies. Experimental studies to date have demonstrated some functional differences between bat species, with the common emerging theme that the overall antiviral response appears to converge on a lower inflammatory profile, with tight regulation of the cytokine and inflammatory response key to clearing viral infection without the pathological outcomes typically associated with infection.
