The protozoa Toxoplasma gondii makes an unobtrusive home in nearly every warm-blooded species, but it's prolific life is limited: Toxo can only reproduce in cat stomachs. The parasite, which causes mild to nonexistant flu-like symptoms, has a clever trick to make sure it winds up where it wants to be. Toxo-infected rats are completely healthy but abnormally attracted to cat urine, and more active than normal. A more outgoing, less fearful rat makes an easy snack for a cat. So far, so good for the parasite's survival strategy, but there's a catch for us. Toxoplasma messes with human brain chemistry in much the same way as it does with rodents.

The effects are sex-dependent. Toxo makes men more distrustful of authority, more jealous, and more likely to engage in rule-bending and breaking. Male motorcyclists are disproportionately affected.  In a perverse twist, motorists of either sex who have T. gondii are three to four times more likely to die in car accidents, either from their increased disregard of the speed limit, or because the parasite wears down reaction times. There's even shaky evidence that T. gondii correlates with success on the football field, at least in predicting the winners of the World Cup.

Women get the sweeter half of the brain parasite. Women harboring T. Gondii are considered by others to be more cheerful, warmhearted, and sexually attractive. They are also outspend their uninfected sisters when it comes to clothing. In some ways Toxo is the microbial mascot of romantic comedies, turning women into spendy social butterflies, and their dates into over-masculine dolts. But take care: Before you go out to find some infectious cat feces to gussy up your social appeal, it's important to point out that the personality changes are statistically significant but still only minor. Researchers still disagree as to how and even if Toxo alters behavior. It could be that the personality predisposes people to the infection, and not the other way around.

While other diseases can cause behavioral changes, Toxo is especially notable for the huge portion of the population it affects – between one third and one half of humanity– and the fact that in healthy people it is largely asymptomatic. The infected can go through their lives blithely unaware that they picked up a disease when they handled raw meat or their cat's litter box. Minor effects may be magnified by three billion people. Some scientists have, with varying degrees of seriousness, attributed differing rates of infection to national character. Between 80 and 90% of France and Germany have it, while the presumably less risk-taking UK and US have only a 7-11% infection rate.

Unfortunately, Toxo is not just some benign little weirdo messing with our choice of vehicle. It can be fatal to people with weakened immune systems and, strangely enough, women infected with T. gondii are more likely to give birth to children that will later develop schizophrenia. The theory that minor early-life infections can trigger schizophrenia, bipolar disorder, or multiple sclerosis is an unsettling addendum to the law of unintended consequences.

Toxoplasma proves that our bodies are not temples: They are cities with porous borders, full of residents temporary and terminal. The personality, the deepest seat of the modern soul, can be tweaked by a parasite originally meant to make rats hyper and horny for cat urine. Our behavior isn't determined just by genes and nurture, and isn't altered through experience or illness alone. Humans could have less latitude in shaping our culture than what we'd like to think.

Given the recent discovery of Toxo's strange symptoms, medicine may still discover a host of other biotic and abiotic pollutants that alter the individual. It may be that we are not just ourselves, but whatever parasites want us to be.

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