Once again, I hear the siren song of Toxoplasma, the parasite that dwells in the brains of 50 million Americans.

Toxoplasma gondii is an extraordinary creature, whose exploits I’ve chronicled in previous posts , an article in the New York Times and my book Parasite Rex. This single-celled organism has a life cycle that takes it from cats to other mammals and birds and back to cats again. Studies have shown that the parasite can alter the behavior of rats, robbing them of their normal fear of cats–and presumably making it easier for the parasites to get into their next host.

Toxoplasma is astonishingly successful, able to live in thousands of species, including us. Billions of people are infected