Though most commonly associated with rust and infections caused by rusty nails, tetanus is actually not caused by rust itself. Rather, tetanus comes from the bacterium Clostridium tetani, the spores for which can often be found on rusty surfaces. The disease is characterized by painful muscle spasms, most often in the jaw (thus the term "lockjaw").
Luckily, the disease can be prevented with regular vaccination. In places where regular tetanus shots are given, such as in the United States, the disease has been nearly eliminated. According to the CDC, only about 30 cases of tetanus are reported in the United States each year, mostly the fault of people late to get their scheduled vaccination.
This photo shows a child after being vaccinated in Lobaye, Central African Republic.