"Toxo," which typically infects cats, is famous among scientists for its clever tactic of jumping from one cat to another by infecting rats and altering their behavior to make them more likely to be eaten by another cat, thus transferring the parasite to a new host.

Flegr discovered that the behaviors that toxo provokes in rats in order to get them eaten—slowed reaction times, lethargy, reduction in fear—also show up in infected humans. But until very recently, scientists knew little about how toxo might be doing this. (Watch: Secret lives of house cats revealed.)

Two months ago, a team of Swedish scientists uncovered a key piece of the puzzle. In order to travel throughout the body and, most importantly, to the brain, toxo