A parasite that lives permanently in the brains of millions may not be as uniformly dormant as scientists once thought. Today, more than a third of the world's human population is infected by Toxoplasma gondii, a brain-invading parasite that reproduces in cats with mice and other animals acting as intermediate hosts. "Our work changes how we think about the Toxoplasma cyst," she adds. "By identifying different parasite subtypes inside cysts, our study pinpoints which ones are most likely to reactivate and cause damage," says Wilson. In the brains of mice chronically infected (for 28 days) with T. gondii, Wilson and colleagues found that cysts contained a greater diversity of parasite subtypes than in the acute stage.