Toxoplasma gondii is the most mind-blowing parasite that I have ever read or heard about. I was driving home when I heard this story on WBUR radio Boston and it really blew my mind. It sounds like an April Fools' science fiction joke. If this doesn't disturb you, you are disturbed.(Just kidding).
So here is the story as presented by WBUR Boston. The 'toxo' parasite I referenced above has a two phased life cycle. A sexual and an asexual part. The sexual part of the life cycle can only take place in the intestines of domestic or wild cats. (felidae family). The asexual part can take place in any warm blooded mammals including humans. It is the sexual part of the life cycle that gets very interesting.
Typically the virus is commonly found in cat feces.( I did not know that rats do this. Mildly disturbing). So a rat eats the stuff and gets infected. The parasite undergoes the asexual part of reproduction in readiness for the next stage. Somehow in order to guarantee its survival, the parasite travels up the rat's bloodstream and into the brain. It infects the rat's amygdala(part of the brain that affects memory and emotional reactions in humans). Amazigly, this infection causes the rat to be attracted to the smell of cats. The rat basically goes looking for the cat, the cat does what cats do and gets infected. Now the 'toxo' can reproduce and the cycle is repeated. I would not have believed this were it not for the reliable sources. Actual scientist have reported this phenomenon.
Another disturbing finding put forth by two independent teams of scientists is that infected humans are 3 to 4 times more likely to die in a car accident.
On the serious side of things the virus can cause fetal deaths in humans and pregnant women are usually advised to avoid cats. The parasite is also linked to alzheimer's disease and scientists are studying whether the domestication of the cat caused the escalation of incidence of the disease in the 19th century.
