Those who believe in free will might be troubled to learn a few secrets about viruses, bacteria and parasites. While it may sound like science fiction, science hints at the potential for microbes to influence our minds, or at least our behavior.

Granted, with very limited exceptions, there’s no conclusive proof that foreign agents can control us from within. But when you consider the evidence with an open mind, it’s interesting to consider the possibilities.

The latest relevant finding seems innocuous enough. Last month, three insect and plant disease researchers in the University of California system reported a discovery about the tomato spotted wilt virus. As its name suggests, this virus infects and damages tomato plants. It’s harmless to people.

To jump from plant to plant, the virus relies on insects known as thrips. A thrip feeds by sticking its oral probe into a plant’s cells and sucking out the contents. If a cell happens to contain the virus, the thrip sucks it up, too.

Scientists already knew that virus-infected tomato plants are more appealing to thrips than uninfected plants. The California researchers discovered something else: Once a thrip consumes the virus, its behavior changes. It spends more time feeding, and it licks more plant cells in the process, coating the next tomato plant with the virus.

(Chris Gash)

The virus’s goal (if viruses had goals) isn’t to mess with the thrip. It only manipulates the insect to get to the next plant. By doing so, the virus is taking away some measure of the thrip’s self-determination. It’s like a fleeing bank robber who commandeers and then abandons a bystander’s vehicle. Car theft wasn’t the criminal’s objective, but the bystander is still deprived.

Scientists have also discovered infections that alter behavior in mammals, including humans. For example, the deadly hantavirus, a distant relative of the tomato spotted wilt virus, causes infected rats to become more aggressive. Rabies, meanwhile, renders its victims crazed and unable to swallow. So rabid bats and canines are more likely to bite and spread the saliva-transmitted virus. In fact, rabies may have provided inspiration for legends of vampires and werewolves. Rabies-infected people don’t tend to bite, but they may foam at the mouth and act belligerently in the infection’s terminal stages.

Not all microbes are so obvious about influencing our behavior. If the effect is subtle, it could be hard to tell whether a behavior is coming from the person or from the thing inside them. Cold viruses, for instance, were recently found to make people friendlier, especially during the period before symptoms appear but when the soon-to-be-sick person is highly infectious to others. Evolutionarily, that helps the virus survive, because a gregarious host is a host who’s likely to spread the illness. Advanced syphilis has been reported to sometimes trigger behavioral changes including an exaggerated desire for sex.

The freakiest of the behavior-warping microbes may be Toxoplasma gondii , the parasite that causes toxoplasmosis. It can live in cats, rodents, people, livestock and other warm-blooded animals, but it reproduces only inside the feline intestinal tract. So the parasite manipulates infected rats, making them attracted to the scent of cat urine when normally they would be repulsed and terrified by it, and causing them to run toward cats instead of away from them. End of rodent. New beginning for parasite.

In some countries, up to about three-quarters of the human population carries toxoplasmosis, which can be acquired by touching cat feces or contaminated soil or by consuming undercooked meat. Normally, only pregnant women and immune-suppressed people get sick. Others develop lifelong “latent” infections, which are symptom-free. Or so it was once thought.

Research in recent years has identified several personality traits that appear to be associated with latent toxoplasmosis. Infected men are more willing to disregard social norms, for example, and are more jealous and dogmatic. Infected women are more conscientious, warm, easygoing and attentive to others. Both sexes, when infected, are more apprehensive and insecure.

Latent toxoplasmosis also slows a person’s reaction time, which may explain why multiple studies have found that infected people get into more car accidents. One prominent researcher speculated that toxoplasmosis indirectly kills a million drivers and pedestrians a year worldwide.

Another researcher summed up the personality patterns by saying that infected men are alley cats — in other words, loners and scrappy fighters — and infected women are sex kittens. A third scientist has hypothesized that the high prevalence of toxoplasmosis in certain countries, including France and Brazil, may influence cultural stereotypes about those nations.

That’s quite a lot to blame on a parasite.

Could other widespread microbes be covertly influencing our individual, or even national, characters? No one knows. The field is a blank slate.

But here’s a possibility to consider: Your body contains roughly 100 trillion cells, only one-tenth of which contain your DNA. The other 90 percent are bacteria. Your body, especially your gut, contains whole worlds of bacteria that live and eat and breed and die within you. Without them, we couldn’t survive; they make vitamins and nutrients that our bodies can’t manufacture on their own.

Last month, an international team of researchers published a detailed analysis of gut bacteria from several dozen people living in different countries. They found they could sort everyone into one of three categories based on which kinds of bacteria were most abundant. They also determined that one bacterial category, or enterotype, is more efficient at making folate, an important vitamin. Another enterotype is better at producing the vitamin riboflavin.

What other differences might exist between people as a consequence of the bacteria that live inside them? Could the microbes be influencing our brains? The discoverers of the three enterotypes didn’t evaluate personality traits, so for now we have no way of knowing.

But my gut would say it’s entirely possible.

Harder is general manager of health and science at U.S. News & World Report.