A loose or exposed nail, weathered by oxidation is the proverbial dreaded source of tetanus, via a puncture wound. The spores of tetanus bacteria are present in soil, dust, and horse manure, but they need an anaerobic environment to germinate, and a puncture wound will do nicely. More than likely the tetanus is already on the surface of the skin, the rusty nail just allows it a means to get inside. So parents warn their children against running around outside barefoot. "Put your shoes on," they say. "Do you want to step on a rusty nail?"

You can also get tetanus with burns, frostbite, and punctures from splinters, insect bites, cactus spines, rose thorns, and knives-- heck, in some countries having your umbilical cord cut at birth puts you at risk for tetanus-- but parents seem to fixate on the "rusty nail = tetanus" meme.