The protozoa Toxoplasma gondii makes an unobtrusive home in nearly every warm-blooded species, but it’s prolific life is limited: Toxo can only reproduce in cat stomachs. The parasite, which causes mild to nonexistant flu-like symptoms, has a clever trick to make sure it winds up where it wants to be. Toxo-infected rats are completely healthy but abnormally attracted to cat urine, and more active than normal. A more outgoing, less fearful rat makes an easy snack for a cat. So far, so good for the parasite’s survival strategy, but there’s a catch for us. Toxoplasma messes with human brain chemistry in much the same way as it does with rodents.