Bats don't respond too much to many viruses etc. They're also tiny living instinctually in hard to find, out of the way, dark places away from predators in massively dense populations. Sometimes just a crack in the rock is enough for a huge bat population.

So because of their low response/death from viruses entire populations in a cave will simply serve as a petire dish while viruses pass freely mutating away as they evolve. Then bats will fly KMs away from there looking for insects etc. Some bats will migrate 100s of miles as well. Huge intense breeding ground combined with wide spread area of infection to spread in makes them super dangerous.

There's a cave in uh, Africa, I believe that is considered one of the most dangerous places on earth. They've traced Ebola among other diseases from that location. It's kind of terrifying.