Harbingers of doom?

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Though they're indispensable to any construction project, nails are so dangerous that it's a wonder that you don't need a permit to buy them at the hardware store. When trying to hammer a nail into its final destination, the risk of hammering a thumb instead is extremely high. Using a nail gun to place the nails isn't any safer; a simple Internet search will procure gruesome stories of nail gun-related injuries. And even when you're safely ensconced within your automobile, a nail can still produce danger and frustration in the form of a flat tire. But to many people, there is no nail more dangerous than the fabled rusty nail. Legend has it that stepping on a rusty nail will cause tetanus.

Tetanus is also known as lockjaw because one of the first symptoms is muscle contraction in the area around the mouth, which leaves the mouth rigidly frozen. Those muscle contractions can spread throughout the body, sometimes resulting in spasms so intense that they cause fractures [source: Brody]. Spasms can also result in difficulty swallowing or breathing, and other symptoms of the condition include drooling, irritability, fever and sweating. These symptoms usually start to appear one week after infection, though they may appear as soon as a few days after, or even a few weeks after infection. Without treatment, one out of three people die from tetanus [source: Medline Plus].

A tetanus vaccine is part of a standard vaccination regime for infants, but its effects can wear off over time. For that reason, adults are urged to get a booster vaccine every 10 years. Because many people stop getting these vaccines as the years go by, older people make up 70 percent of reported cases of tetanus [source: Brody]. But is a rusty nail the culprit? If so, why? And if not, what causes tetanus?