Tetanus is often associated with rust, especially rusty nails, but this concept is somewhat misleading. Objects that accumulate rust are often found outdoors, or in places that harbor anaerobic bacteria, but the rust itself does not cause tetanus nor does it contain more ''Clostridium tetani'' bacteria.
The rough surface of rusty metal merely provides a prime habitat for a ''C. tetani'' endospore to reside, and the nail affords a means to puncture skin and deliver endospore into the wound.
An endospore is a non-metabolising survival structure that begins to metabolise and cause infection once in an adequate environment. Because ''C. tetani'' is an anaerobic bacterium, it and its endospores survive well in an environment that lacks oxygen. Hence, stepping on a nail (rusty or not) may result in a tetanus infection, as the low-oxygen (anaerobic) environment is provided by the same object which causes a puncture wound, delivering endospores to a suitable environment for growth.
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