Toxoplasmosis

Toxoplsma gondii is a parasite related to the malaria parasite and can be carried by many warm-blooded mammals including humans. But its definitive host is cats and it can sexually reproduce only in cats. Its eggs are spread in cat faeces. A passing rat can pick it up from the moist infected soil. Apparently rats behave recklessly when infected (resulting in toxoplasmosis or toxo for short) , are more active and less scared of new things and are even attracted to cat's urine. This makes them easy prey to cats and the life cycle of the parasite continues.
Between 30 to 70 percent of the humans (depending on the country) are supposed to be infected with this parasite and the effects may vary considerably. Apparently it can affect one’s intelligence, has been linked to schizophrenia and may raise chances of being knocked out by a car. But the main danger seems to be to pregnant women and pregnant women are urged to be vary of handling cats and cat litter:
http://www.cdc.gov/ncidod/dpd/parasites/toxoplasmosis/factsht_toxoplasmosis.htm
According to New Scientist:
http://www.newscientist.com/article/mg17623665.200;jsessionid=GDHALMBFEDLA
the definite host is cats and the effects on humans may be much less harmful and depend on the gender:
“The results from personality tests were complicated and showed confusing gender differences, but men at least seemed to mimic one aspect of rat manipulation. Infected men tended to be more independent and inclined to break rules, although infected women tended to go the other way. Could it be that males are being made more reckless, like the rats, while for some reason the mind-control chemical has the opposite effect in females? “
“But one test, measuring reaction times and attention span, gave more consistent results (Parasitology, vol 122, p 515). Both men and women who had a latent infection took longer to press a computer key after a prompt from the monitor. People without an infection took about 250 milliseconds to react, but those with a latent infection reacted about 8 per cent slower. What's more, toxo-positive subjects did worse as the experiment went on, suggesting that they have shorter attention spans. Again, the effects in humans seem to be mimicking those in rats.
Until recently, few people have taken Flegr's results very seriously. So what if toxo causes a few strange lab results, what difference does that make in the real world? What made people sit up and take notice was research published in August by Flegr's team showing that humans with a latent infection are 2.7 times more likely to be involved in a car accident (BMC Infectious Diseases, www.biomedcentral.com/1471-2334/2/11).”
It is possible that this parasite has very little affect on humans but I was reminded of it by the developments in the middle east.
Update (August 2, 2006). See http://scienceblogs.com/loom/2006/08/01/a_nation_of_cowards_blame_the.php
for the latest.