bats have an unusual immune system:

https://www.sciencenews.org/article/bats-immune-system-virus...

Bats get around (they can fly!) and they're mammals, so the viruses they carry are somewhat compatible with us. Not that birds aren't also virus spreaders (West Nile, etc.).

There's much more to it, of course.

https://www.washingtonpost.com/science/why-do-bats-have-so-m...