Common Intestinal Residents

Pinworms spread through one of the simplest cycles in nature: eggs laid around the anus cause itching, scratching moves eggs onto the fingers, the fingers touch food or door handles, and a new host swallows them. That is why pinworms race through households and classrooms.

Toxoplasma gondii is stealthier. It forms quiet cysts in the brain of about a third of all people, usually picked up from undercooked meat or cat litter, and most carriers never notice.