I've heard it's because of their rare status as a flying mammal. In short, keeping their weight aloft when they aren't as well adapted to flying as, say, birds which have hollow bones, means they have to be exceptionally tough, and this includes having a highly robust immune system.

Robust immune system means they are ideal carriers because they can contact loads of viruses without becoming incapacitated, and coupled with their ability to cover reasonable distances they have the opportunity to spread them.