Well, nature has a way of mutating strains of viruses. Just look at all STD's for example, and even the black plague and smallpox. 
It wouldn't be that crazy for a rabies virus to suddenly mutate to a super high strength version. Many viruses do it all the time, and seeing as how rabies is highly infectious, capable of crossing many species (pretty much all mammals) and is a very "base" type of virus with a few effective way of spreading itself (through other animals attacking other animals, crazy how that works out huh?), I'd say it's long over due for a mutation. 
Coupled with the fact that we've mostly, but not entirely eliminated it in the U.S., means that very potent or powerful strains are waiting around to infect someone. Viruses can't die either. O_o
But it would be cooler if worms did it. 
Zombie Snails.Zombie antsThere are several other types of zombie parasites. O_o