There are actually viruses that could have this kind of effect.
http://www.sciencemag.org/content/265/5 ... 5.abstract
Say a pandemic virus that targets this particular receptor in the human brain, for example. It would be just like the plot of "28 days". ]
Additionally there are a number of parasitic organism that can alter or even control the behavior of their host.
So while it may seem far fetched idea, it really isn't.