Tetanus is a dangerous nerve ailment caused by the toxin of a common bacterium, Clostridium tetani. Bacterial spores are found in soil -- most frequently in cultivated soil, least frequently in virgin soil. The spores can remain infectious for more than 40 years in soil. They also exist in environments as diverse as animal excrement, house dust, and the human colon. If the spores enter a wound that penetrates the skin and extends deeper than oxygen can reach, they germinate and produce a toxin...
You are bitten by an animal or wounded by an object that might be contaminated with dirt, feces, or dust, and you have not been immunized against tetanus or received a booster within the last five years. Tetanus infection can be fatal and should be treated as soon as possible.
CDC: "Tetanus (Lockjaw) Vaccination."
National Network For Immunization Information: "Diphtheria, Tetanus, Pertussis (DTaP)."
National Association for Infectious Diseases: "Tetanus (Lockjaw)."