Since we have alot of zombie related topics here recently i would like to introduce the Toxoplasma gondii parasite to you. They have an extremly complex life cycle that goes through many hosts. It is able to change the behavior of its host. One example for that are infected rats. They stop to be shy animals and walk on open places to attract predators. Recent research also suggests they go actively to places where cats are dominant. How that change in behavior is achieved is still not known. Toxoplasma gondii can also infect humans. While it can create some serious symptoms in most cases it stays mostly unnoticed. A high percentage of hosts develop some kind of schizophrenia. It seems, that the parasite does something with the mind of his host. But it is still unknown what exactly is going on. Almost all people who are infected by this parasite tend to be calm, introverted and conservative. The highest infection level world wide is in switzerland. Psychologial tests indicate that the infection leads to massive changes in personality the longer the person is infected. Men start to mistrust others and care less about rules while women start to become more caring and warmhearted. Both genders create more accidents. Both genders tend to feel more guilty, are unsecure and nervous. But why should Toxoplasmosa manipulate a humans brain? We are no food scource for cats. Scientists say we aren´t today but there were times we were a food scource for big cats. That Toxoplasmosis slows down the reaction of its hosts is also noticed by humans (see more car accidents). All in all that parasite and maybe others could play a very strong role in human history.
http://www.zeit.de/zeit-wissen/2009/02/Toxoplasmose
http://www.zeit.de/zeit-wissen/2009/02/Toxoplasmose