Tetanus: An often fatal infectious disease that is caused by the bacterium Clostridium tetani, which usually enters the body through a puncture, a cut, or an open wound. Tetanus leads to profound painful spasms of muscles, including 'locking' of the jaw so that the mouth cannot open, and death. The C. tetani bacteria releases a toxin that affects the motor nerves, which stimulate the muscles. Prevention involves immediately cleaning and covering any open wound and getting a tetanus vaccination. Regular boosters are necessary to ensure immunity. Unvaccinated people who get puncture wounds or cuts should get tetanus immunoglobulin and a series of tetanus shots immediately; those who have been immunized but are unsure of the date of their last tetanus shot should get boosters. Also known as lockjaw.