Rat-bite fever is at least two, and possibly three, bacterial diseases that in humans are clinically similar yet distinct syndromes if allowed to run their courses. Rat-bite fever due to Spirillum minus, designated spirillary rat-bite fever, spirillary fever, spirillosis, or sodoku, which is the Japanese word meaning rat poison, is a separate disease. Rat-bite fevers are systemic, generally acute, febrile illnesses. They begin with sudden onset of fever, chills, and headache following the bite of a rat, rarely other animals, and after nonbite exposure. The usual mode of transmission for streptobacillary and spirillary rat-bite fever to humans is by a bite from a carrier animal, usually by rats or mice, but cases from bites of squirrels, weasels, and gerbils also have been reported. Prevention of streptobacillosis and spirillosis primarily depends on reducing the risk of exposure to rat bites and, in the case of nonbite disease, on reducing rat contamination of human environments, food, and water.