Tetanus is a serious disease caused by bacteria that produce a powerful toxin that attacks the nervous system. The bacteria live in soil, dust, and animal waste, entering the body through cuts, punctures, or wounds. Once inside, they release a toxin that causes painful, uncontrollable muscle spasms, especially in the jaw and neck. This jaw stiffness gave tetanus its common name: lockjaw.

Before vaccines, tetanus killed thousands of people every year. A farmer stepping on a rusty nail, a child scraping a knee in the dirt, or a soldier wounded in battle could all develop tetanus. The disease was particularly dangerous because even a tiny, seemingly harmless wound could let the bacteria in.

Today, doctors prevent tetanus with a vaccine that trains your immune system to fight the toxin. Children receive tetanus shots as part of their regular vaccinations, with booster shots needed every ten years throughout life. When someone gets a deep cut or puncture wound, especially from something dirty or rusty, doctors often give a tetanus shot as a precaution if the person's vaccination isn't current.

The rusty nail association comes from the fact that puncture wounds, which rusty nails often cause, create the airless environment tetanus bacteria need to thrive. But the danger comes from the bacteria in dirt on the nail, not the rust itself.