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There’s a pretty good chance that you have a parasite in your brain.

Don’t let it freak you out or anything—after all, you’re not alone. About 30-50 percent of people have been exposed to Toxoplasma gondii, the protozoa that cause a condition called toxoplasmosis.

T. gondii is so common, in fact, that doctors often warn pregnant women to take precautions to prevent infection. The parasite can present serious issues for immunocompromised persons, and because it can infect just about any warm-blooded animal, it’s not going away anytime soon.

Oh, and some studies have suggested that T. gondii can control behavior. There are some very important caveats to consider when approaching that fact, but first, let’s look at how humans come into contact with this nefarious parasite.

Nearly any animal can become infected with T. gondii, but only one animal provides the environment that the organism needs to reproduce.

That animal would be the domestic housecat. Yes, the same adorable little creature currently living in your home.

But T. gondii doesn’t always infect the housecat directly, as it’s ill-suited for that task. Instead, it typically infects their prey in a process called secondary infection.

Here’s what happens: The parasite infects a mouse, then works its way into the rodent’s brain, changing neural pathways to make the mouse seek out cats (or at least show less caution when near felines).

Cats that eat these rodents become infected, allowing T. gondii to reproduce. The protozoa find new hosts by being released via the cat’s feces (sorry, but you had to know that a story about a brain-altering parasite wouldn’t be too clean).

Humans often become infected when cleaning up after their housecats. They can pass the parasite on by not properly washing their hands before preparing food. While T. gondii can’t reproduce in our bodies, it can live for quite a while—and while it’s hitching a ride, it might change our personalities.

Various studies show that T. gondii infection can prompt behavioral changes in humans.

Some have even associated the protozoa with neurological disorders like schizophrenia. Studies like “Toxoplasma gondii–induced neuronal alterations,” written by A. Parlog et al, claim that the parasite can affect synaptic plasticity and neuronal connectivity. In other words, toxoplasmosis might literally change the way your brain works.

Another study showed that T. gondii increased levels of several chemicals in mice brains, including kynurenic acid (KYNA). That acid is linked to schizophrenia in humans. Other studies have indicated links between depression and toxoplasmosis.

If you’re looking for some good news, we’ve got some; an analysis of the massive Dunedin Longitudinal Study showed no notable link between toxoplasmosis and any personality traits, nor with rates of neurological disorders.

This obviously conflicts with the information presented in earlier studies, but as the researchers note, earlier analyses worked with much smaller sample sizes. It’s possible that the protozoa can’t control your brain—unless, of course, you’re a mouse.

Then again, that might just be what the T. gondii wants us to think.