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.