Eons of evolution have embedded a natural fear of predators in the minds of all creatures. That's why when a chimp sees a leopard, the chimp will run away. And so would we all. However, there is a parasite found in cat feces, called toxoplasma gondii, that causes the opposite reaction to occur when a creature (in this case, a chimp) is faced with real danger.
Researchers in France and Germany analyzed the way in which a group of chimps infected with toxoplasma gondii reacted to leopard urine, as opposed to an uninfected control group, and also as opposed to other types of urine. Namely, human, tiger, and lion, as none of these creatures are enemies of the chimpanzee. On the other hand, about 30 percent of chimpanzee deaths are caused by leopards. Scientists found an interesting correlation. The infected chimps were not afraid of the leopard urine, some of them even moving in closer to get a better sniff at it. The uninfected chimps kept their safe distance.
When a feline eats an animal that is infected with T. gondii, the parasite lives out the rest of its life in the cat's digestive system. There, it will reproduce, and the offspring will eventually escape by way of the cat's feces. Then, smaller animals will eat the baby parasites. Then cats will eat the smaller animals, and the cycle starts all over again.
This particular parasite affects the brain in order to influence behavior. To kill the fear factor. This boosts the odds of successful reproduction of the parasite. Studies that were conducted on rats and mice proved this point decades ago. Rats (or mice) who were infected with T. gondii showed a curious nonchalance around the formerly dreaded smell of cat urine. This time, no flight response was triggered. In fact, in some cases, there may have been a sexual attraction to the cat urine. Uninfected rats and mice did as one would normally expect. They skittered away.
This abject lack of fear in the face of real danger increases the odds that chimps who are infected with T. gondii will be eaten. And this will start the parasitic cycle all over again.
In humans, T. gondii can be acquired from eating raw meat, drinking contaminated water, and even from the litter box of a domestic household cat. A study conducted in 2007 linked T. gondii with impaired concentration and reaction times in humans. This could affect a person's driving skills, for example.
The Centers for Disease Control and Prevention (CDC) estimates that roughly one in 60 million people in America is infected with T. gondii. Wait. That's only about five people. That can't be right. But that's what Discover Magazine says, so who am I to argue?
In any case, according to the CDC, the infection is said to be harmless and most often goes unnoticed. But in people with weakened immune systems, signs of T. gondii can include flu like symptoms, altered risk behaviors, and even schizophrenia.