In my scenario, people behave after a certain "transformation" similar to Zombies. However they are not "undead", meaning that they cannot raise again after their body functions completely are out of order. Their behavior resembles those of zombies in the following way:
Not capable of having verbal interaction with non-transformed humans
Hostile towards every mammal which is directly encountered
Regards human flesh as nutrition like flesh of any other mammal
Is it possible that a transformation which leads to this behavior can be caused by substances that possibly exist nowadays, probably also in nature?
One option I thought of: Long exposure to a certain poisonous gas in an abandoned mine. The exposure to the gas damages sections in the human brain permanently, which causes humans to lose any common sense, behave like rabid animals and triggering them to regard every living being as possible nutrition.