In a study of 12- to 49-year-olds who were born in the U.S., about 9 percent were found to be infected with T. gondii in the early 2000s, according to the Centers for Disease Control and Prevention. That rate is down from 14 percent in the 1990s. However, Yolken believes infection rates are higher among older Americans.

Yolken, a cat owner who says he has tested positive for Toxoplasma antibodies, said the potential link between Toxoplasma infections and mental illness is no reason for cat owners to panic - they just need to keep some basic hygienic precautions in mind.

He was drawn into the field by some intriguing questions about schizophrenia.

"I couldn't understand why a disease like schizophrenia persists in humans," he said. Through much of our history, "people who have these diseases don't reproduce very well, either because they're sick, or they've been locked up, or because they were killed."

If the disorder were strictly genetic in origin, he added, those genes should have been culled from the gene pool long ago. But they weren't. That raises the question of an environmental, perhaps infectious origin - a germ that has evolved to benefit by infecting other species.