Could microscopic parasites have the ability to take control of a human being? Scientists are starting to think so.

A third of the world's population may have a parasitic infection that scientists believe to have an impact on human behavior.

The protozoan parasite, called Toxoplasma gondii, has long been considered to be an "asymptomatic" parasite in humans. But lab tests and a new report suggest that it may cause or intensify severe forms of schizophrenia, could have an impact on how human hormones are secreted in the brain, and may cause personality changes.

Scientists have determined that the parasite, which thrives in rats and reproduces in house cats, tricks rats into getting into harm's way.

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"The parasite grows in a rodent, but it needs to get into a cat somehow to reproduce," says Shelley Adamo, a biologist who studies neuroparasitology at Toronto's Dalhousie University. "When a rat becomes infected, the parasite somehow makes rats become attracted to cat urine, when it would normally avoid it."

The CDC estimates that more than 60 million Americans carry the single-celled parasite. Most people get it from infected, undercooked meat or from cats. According to the agency, "of those who are infected, very few have symptoms because a healthy person's immune system usually keeps the parasite from causing illness." But scientists are starting to rethink that theory.

In a study published Wednesday in the Journal of Experimental Biology, Jaroslav Flegr, a scientist at Charles University in Prague, suggests that people infected with Toxoplasma have slower reaction times and are "less altruistic" than non-infected people.

Women infected with Toxoplasma "more often report that diplomacy is not their strong point...that some people have the power to impose their will on others with hypnosis... and that they have a weak instinct for self-preservation: in situations where somebody else might be afraid, for example being alone in a forest or in an empty house at night, they remain calm."

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Flegr notes that the existence of these correlations "cannot distinguish whether the observed changes are manifestations of the manipulative activity or only symptoms of the chronic disease" that is sometimes caused by Toxoplasma. But he says that the widespread prevalence of Toxoplasma infection makes it an ideal "model for studying manipulative activity in humans." He says there are a "large number of parasitic organisms ... that may influence the human host even more than the Taxoplasma."

Most neuroparasitologists, like Adamo, normally study insects, where there are numerous examples of insects infected with parasites behaving oddly. The most famous examples involve "zombie caterpillars," which are controlled by a parasitic virus that instructs the caterpillar to climb to treetops where they melt in the hot sun, raining the virus down on other potential hosts.

Even in insects, it can be hard to study the parasite-host relationship because of the need to raise two organisms in a lab and facilitate an interaction, Adamo says. In humans, it's nearly impossible.

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"The ultimate experiment would be to have a bunch of people, and to infect some of them, and then measure their behavior before and after," Adamo says. "We're never ever going to be allowed to do that with humans." In a lab setting, Toxoplasma also binds to dopamine receptors, which are the brain's reward centers.

"It's quite possible and plausible that if you have an agent affecting dopamine, it could influence people's behaviors," Adamo says.

A disease that completely alters a human's behavior isn't unprecedented. A human infected by rabies will nearly always display a fear of water, just the way a dog would--so it's not beyond the realm of possibility that a parasite could influence a human's behavior in more subtle ways.

"In invertebrates, the effects are far more drastic. In mammals, the effects are smaller. I say that, but then you look at something like rabies," Adamo says. "And you see that things like this can cause huge changes in behavior."

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Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at jkoebler@usnews.com.