On paper, Toxoplasma gondii looks as if it ought to be the most famous parasite on earth. This single-celled pathogen infects over half the world's population, including an estimated 50 million Americans. Each of Toxoplasma's victims carries thousands of the parasites, many residing in the brain. As if that were not enough of an accomplishment, Toxoplasma is equally adept at infecting all other warm-blooded animals, as disparate as chickens and kangaroos. Scientists are now discovering some of the secrets of Toxoplasma's success. Researchers in Sweden report that the parasite fans out through the body by manipulating mobile cells that are...