Toxoplasma is a parasite which alters the mind(behaviour) of its host.

From some site:

A recent study has indicated toxoplasmosis is also correlated strongly with an increase in boy births in humans, leading to an alteration of the human sex ratio.

Other possible behavior modifications are suggested by a study suggesting that people not infected with the parasite found women with toxoplasma more attractive than women who don't have toxoplasma.

"Interestingly, the effect of infection is different between men and women,'' Dr Boulter writes in the latest issue of Australasian Science magazine.

"Infected men have lower IQs, achieve a lower level of education and have shorter attention spans. They are also more likely to break rules and take risks, be more independent, more anti-social, suspicious, jealous and morose, and are deemed less attractive to women.

"On the other hand, infected women tend to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with non-infected controls.
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Another article on toxoplasma:

Mind Control by Parasites
-By Bill Christensen


And another:

Study: Cat Parasite Affects Human Culture
http://www.livescience.com/strangenews/060803_tgondii_culture.html

excerpt:

{ Studies by researchers in the Czech Republic have suggested T. gondii might have subtle but long-term effects on its human hosts. The parasite is thought to have different, and often opposite effects in men versus women, but both genders appear to develop a form of neuroticism called "guilt proneness."
Other studies have also found links between the parasite and schizophrenia T. gondii infection is known to damage astrocytes, support cells in the brain that are also affected during schizophrenia. Pregnant women with high levels of antibodies to the parasite are also more likely to give birth to children who will develop the disorder. }