On the trail of a stealthy parasite
About one-third of the human population is infected with a parasite called Toxoplasma gondii, but most of them don’t know it. Though Toxoplasma causes no symptoms in most people, it can be harmful to individuals with suppressed immune systems, and to fetuses whose mothers become infected during pregnancy. Particularly dangerous strains, found mainly in South America, are the leading cause of blindness in Brazil.Toxoplasma is one of the very few parasites that can infect nearly any warm-blooded animal. Its spores are found in dirt and easily infect farm animals such as cows, sheep, pigs and chickens. Humans can be infected by eating undercooked meat or unwashed vegetables.“It’s everywhere, and you just need one spore to become infected,” says Jeroen Saeij, an assistant professor of biology at MIT. “Most cases don’t kill the host but establish a lifelong, chronic infection, mainly in the brain and muscle tissue.”Saeij is investigating a key...