Tetanus is caused by bacteria, called Clostridium tetani, which are normally present in the soil. If the bacteria enter a wound, they start to grow and produce a toxin. The toxin is transported by the bloodstream and lymphatic tissues to nerves.

The toxin blocks the action of nerves that block muscle contractions. In turn, this allows other nerves to constantly stimulate the muscles to contract, resulting in painful, severe muscle spasms. Once the toxin attaches to a nerve, the toxin cannot be neutralized by antitoxin.