Tetanus bacteria, illustration. Clostridium tetani is a gram-positive, spore-forming, anaerobic bacillus (rod-shaped bacterium) that causes tetanus. Members of the Clostridium genus exist as soil saprophytes and as intestinal parasites of animals, giving rise to serious human diseases, including botulism, gas gangrene and severe intestinal infections, in addition to tetanus. Incidence of tetanus is common in societies where bare-footed humans live in close association with cattle and other domesticated animals, although the disease may result from trivial wounds to the skin.