Many people could already be infected with this zombie disease...

Puncake32

This disease doesn't effect humans what so ever, but many humans do have this disease one way or another...
https://en.wikipedia.org/wiki/Toxoplasma_gondii

It can only effect rats, but if the virus somehow evolves, it can effect humans and those effected could become a zombie pretty quick. I am most likely already effected, Anyway, I found that this is quite interesting and thought I should share it.