When executive producer Ellen Horne was expecting a baby, she really had no particular intention of becoming a self-made expert on a parasite named Toxoplasma gondii. Robert Sapolsky explains to us why Ellen had reason to worry when she was scratched by her cat, and he traces the unlikely path that the parasite might follow, right up to the point that it rewires a rat's brain. Fuller Torrey details Toxoplasma's potential associations with other human disorders, possibly even schizophrenia.