When I met Flegr for the first time, last September, at his office on the third floor of Charles University’s Biological Sciences building, I was expecting something of a wild man. But once you get past the riotous red hair, his style is understated. Thin and slight of build, he’s soft-spoken, precise with his facts, and—true to his Toxo status—clad in old sneakers, faded bell-bottom jeans, and a loose-fitting button-up shirt. As our conversation proceeds, I discover that his latest findings have become—to quote Alice in Wonderland—“curiouser and curiouser,” which may explain why his forehead has the deep ruts of a chronic worrier, or someone perpetually perplexed.

He’s published some data, he tells me, that suggest infected males might have elevated testosterone levels. Possibly for that reason, women shown photos of these men rate them as more masculine than pictures of uninfected men. “I want to investigate this more closely to see if it’s true,” he says. “Also, it could be women find infected men more attractive. That’s something else we hope to test.”

Meanwhile, two Turkish studies have replicated his studies linking Toxoplasma to traffic accidents. With up to one-third of the world infected with the parasite, Flegr now calculates that T. gondii is a likely factor in several hundred thousand road deaths each year. In addition, reanalysis of his personality-questionnaire data revealed that, just like him, many other people who have the latent infection feel intrepid in dangerous situations. “Maybe,” he says, “that’s another reason they get into traffic accidents. They don’t have a normal fear response.”

It’s almost impossible to hear about Flegr’s research without wondering whether you’re infected—especially if, like me, you’re a cat owner, favor very rare meat, and identify even a little bit with your Toxo sex stereotype. So before coming to Prague, I’d gotten tested for the parasite, but I didn’t yet know the results. It seemed a good time to see what his intuition would tell me. “Can you guess from observing someone whether they have the parasite—myself, for example?,” I ask.

“No,” he says, “the parasite’s effects on personality are very subtle.” If, as a woman, you were introverted before being infected, he says, the parasite won’t turn you into a raving extrovert. It might just make you a little less introverted. “I’m very typical of Toxoplasma males,” he continues. “But I don’t know whether my personality traits have anything to do with the infection. It’s impossible to say for any one individual. You usually need about 50 people who are infected and 50 who are not, in order to see a statistically significant difference. The vast majority of people will have no idea they’re infected.”

Still, he concedes, the parasite could be very bad news for a small percentage of people—and not just those who might be at greater risk for car accidents. Many schizophrenia patients show shrinkage in parts of their cerebral cortex, and Flegr thinks the protozoan may be to blame for that. He hands me a recently published paper on the topic that he co-authored with colleagues at Charles University, including a psychiatrist named Jiri Horacek. Twelve of 44 schizophrenia patients who underwent MRI scans, the team found, had reduced gray matter in the brain—and the decrease occurred almost exclusively in those who tested positive for T. gondii. After reading the abstract, I must look stunned, because Flegr smiles and says, “Jiri had the same response. I don’t think he believed it could be true.” When I later speak with Horacek, he admits to having been skeptical about Flegr’s theory at the outset. When they merged the MRI results with the infection data, however, he went from being a doubter to being a believer. “I was amazed at how pronounced the effect was,” he says. “To me that suggests the parasite may trigger schizophrenia in genetically susceptible people.”