Rabies is a disease caused by a virus. It is not a defense mechanism caused by some sort of "venom" they carry. Bats with rabies DIE of rabies.
Unfortunately the real situation seems to have conflated with the Dracula legend so now if a bat bites a person he goes crazy and bites two people, they go crazy and bite four people, they go crazy and bite eight people and so the corruption spreads throughout the vorld!!!