In Flegr's research, men infected with T. gondii were, on average, more antisocial, prone to fights and less intelligent than their parasite-free counterparts, but were also more reflective, organized and were not easily distracted. Infected women were more easygoing, cared more about their appearance and had a higher level of intelligence than their control group, but were also less trustworthy and possibly more promiscuous. Every character trait Flegr assessed progressed in opposing directions between the infected sexes, except one: an increased tendency toward self-reproach. Also, both men and women, Flegr writes in a study published in July, "have significantly deteriorated psychomotor performance (prolonged simple reaction times)...and are at higher risk for traffic accidents."

"I think this warrants more study," says Dr. Philip Tierno, director of clinical microbiology and diagnostic immunology at the NYU Medical Center. "But I certainly wouldn't mind cooperating with a group. The parasite has a predilection for the central nervous system and the brain; the cysts seek out brain tissue. So the concept of it affecting personality is intriguing."

Flegr hypothesizes a connection between the character alterations he found and changing levels of dopamine (a neurotransmitter naturally produced in the body) in brains inhabited by the parasite. In an earlier study on infected mice, typical Toxoplasma hosts, increased dopamine levels impaired motor performance, impeded their ability to remember their environment and increased attraction to nonfamiliar environments—as a result, the parasite made them more likely to be eaten by cats, the only animal in which T. gondii can reproduce.

It's common for parasites to body-snatch their hosts. For example, hairworms cause grasshoppers to commit suicide by drowning, since the worms can only exit the insects in water. Theories about intentional human manipulation by T. gondii pop up on the Internet—e.g. aggressive men could be killed, leaving their flesh to be eaten by cats, or animals which in turn could be eaten by other cats. But Flegr and his team aren't ready to commit to the theory—especially since humans aren't natural hosts. Although he doesn't conclude either way, he argues that personality changes are likely side-effects. His work suggests that human behavioral changes may not be apparent immediately and will increase over time, further supporting the notion that the parasite isn't running the show—not for its own gain, at least.

Although human infection by these protozoa can always be traced to cat feces, the good news is that it probably isn't from your cat. About half of U.S. infections, according to the United States Department of Agriculture, come from eating the raw or under-cooked meat of animals exposed to contaminated feline fecal matter (typically through water, soil, feed or fertilizer). In fact, 84 percent of the French population is infected, which Tierno believes is due to another predilection: theirs for steak tartare. In addition to avoiding raw meat yourself, don't feed it to your cat. 

And don't let them hunt: Most acquire T. gondii carnivorously through rodents, birds and other small animals. New Yorkers don't typically let their cats outside, which helps, but if your kitty's a mouser, seek out and plug up rodent holes anyway. If you're feeling charitable toward one of the numerous communities of strays in the city's empty lots, leave food at a distance from their stomping (and dropping) grounds. And if you take in a stray, make sure to clean the litter box twice a day in case he's infected: Once the parasite has reproduced, the eggs (or oocysts) are excreted in the cat's waste for two to three weeks, but it takes 24 hours for them to become contagious once expelled. They'll remain active, however, for months and can live in soil for up to a year; always wash your hands after gardening.

Many city kitties live in, came from or are remotely monitored by shelters. Of the ten rescue organizations we called, none test for Toxoplasma routinely, but all will do so by request. Tests—although relatively easy and inexpensive—are inconclusive. Cysts can remain in feline tissue forever, but a few months after initial infection, the cat is no longer contagious. "We test for the presence of antibodies [secreted by the cat's immune system to fight the parasite]," says Dr. Lori Bierbrier of Hope Veterinary Clinic in Boerum Hill, Brooklyn. "So it can tell us that the cat has been exposed, but not when. The results are a can of worms." In fact, if your cat tests positive, that could be good news. "Reinfection causes little to no egg-shedding," explains Dr. Elaine Felton, medical director of the Brooklyn Animal Resource Coalition. In other words, once a cat gets Toxoplasma gondii, it can't really get it again. "So a cat with a positive test result, since it's probably not contagious any more, poses less risk to you than one which is free but could be infected later," she says and adds with a laugh, "unless you eat your cat."

Of pet owners in New York City, 60 percent have cats—and if they take the right precautions, and avoid raw meat, they'll be completely safe from infection. As for the millions who've already been invaded by this accidental body snatcher? "Technically, the die is cast," Tierno says. "Perhaps as research goes on, individuals will develop a drug that can get into the brain and pinpoint Toxoplasma. But at this point in time, I don't know of any medication to prophylactically treat aggressive behavior."