There are a number of parasites that radically alter the behavior of their hosts once infected, sometimes in surprising and dramatic ways.
Ophiocordyceps unilateralis is a fungal parasite that requires very specific conditions in which to live. In order to ensure that it develops in these conditions the parasite infects a species of ant and makes its host seek out and attach itself to a leaf in exactly the right conditions that the fungus needs in order to survive. The fungus then kills the ant and continues to develop until its fruiting body grows out of its victim’s head, eventually exploding and releasing thousand of thousands of spores.
The case of Toxoplasma gondii, however, is even more exceptional. The reproductive stage of this parasite’s life cycle can only be completed inside the stomach of a cat, but it can infect almost any warm-blooded mammal. When it infects a rodent, it actually destroys its fear of the smell of cats. Not only does the hapless rodent no longer fear cats, but it becomes attracted to them, and encourages riskier behaviour in the intermediary host.
And that’s not all. Toxoplasma gondii can infect humans as well. The behavioral changes are still being reviewed, but could be substantial. Nicky Boulter, a researcher from Australia has commented that:
"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. In short, it can make men behave like alley cats and women behave like sex kittens."
This is of exceptional interest because at any given time a substantial portion of the popular (around 40%) are infected by this parasite since its primary host is such a common household pet.
For more information on Ophiocordyceps unilateralis:
And for Toxoplasma gondii: