A forced partnership between parasite and host allows Toxoplasma gondii to invade the bloodstream, break into the brain, and prompt behavioral changes from recklessness to neuroticism. The highly contagious protozoa that infects more than half the world’s population evades the body’s defenses by hacking immune cells, making it the first known parasite to control its host’s immune system. Most of those infected never purge the intruders.
