Toxoplasma gondii is a parasite that can make any mammal its host, but can make eggs only when in a cat's body. Cats pick it up by eating infected meat. Once the parasite reproduces, an infected cat can spread millions of eggs or "oocysts" in its feces for 10 to 14 days.
Although an estimated one-third of the human population may harbor Toxoplasma, only a small number will ever show any clinical symptoms. Still, it does pose a threat to pregnant women, since it can pass the placental barrier and increase the risk of miscarriage.
Rats that get infected become more prone to risky behavior, making them more likely to wind up as prey for cats - and thus helping spread the parasite. However, though studies have indicated a link between toxoplasma infection and psychological problems in humans, these results are tentative.