It's actually not as far fetched as one might think. There are several known fungi and protozoa that already effectively make zombies out of living creatures. Look up toxoplasmosa gondii for one. It can be carried by most warm blooded animals, including humans... It makes rats actually go toward cats, so they will get eaten and the protozoa can reproduce in the cats intestines. The protozoa can only reproduce in feline guts... The scary part is about 1/3 of humans carry this protozoa too, and it's changing. It's actually already showing up in studies related to human suicide. You think I'm kidding? Look it up.
That doesn't even touch on the neurotoxin possibilities too! Let the zombie hoards begin!