There are many different types of bats. They all don't eat the same things.
Some bats eat insects.
Others eat fruit. There's a great number of things that they eat. The vampire bat eats blood from other animals by biting it and then sucking the blood that comes out, the vampire bat's saliva has chemicals that keep the blood flowing so that it won't have to bite the same animal several times to get enough food. Because vampire bats suck blood from cattle and other large animals it is possible that it could get a disease from the blood. If the bat does get a disease it would be possible for it to transmit it from one animal to another.