Poisoning, Intoxication, Drug Influence etc. - Substances can change the way a person behaves. Picture a chemical that can actually cause humans to act and do things they wouldn't normally do. This could cause a sort of epidemic if chemical warfare happened.
Virus, Infection, etc. - The method you most likely hear about. Though viruses like Rabies if airborne could cause widespread madness in human beings. Another thing which would cause you to do things you wouldn't normally do. The odd thing about Rabies is also that it
might be a cure for cancer. That really brings the movie I am Legend to reality.
Psychological Imbalance/Neurological Disorders/Brain Damage - If a genetic defect is severe enough it could cause behavior which is not typical of acceptable human behavior. Though with something like this to a degree, humans can still learn and be trained to act in a sort of acceptable way. Enough to manage to a degree not to resort to cannibalism.
Experiments -
Zombie Cells are being created in many places here in the US. If someone is introduced to some types of cells and it takes a turn for the worst, you never know what could happen. We could ultimately cause our own end by just being curious.
Gives you something to think about doesn't it? Science Fiction in a way isn't so far from the truth.