bats is very mysterious, i think its due to thier long evolutionary history compared to most mammals, allow them to co-exist with the pathogens. they suddenly came into the picture right before the cretacaeuous ended, we dont even know what bats evolved from. funny thing one of the clues is "bat bugs" which are predecessors to bed bugs( and they can infect humans). other unusual mammals are solenedons/nosphenthes lineages.