Tetanus is an infection caused by a bacterium called Clostridium tetani which can attack the muscles and nervous system. Tetanus is a serious infection which can even be fatal. Tetanus bacteria (germs) live in the soil and dirt. The bacteria may get into your body through a cut or a wound in the skin. The bacteria make a toxin (poison) which causes the illness.
Even small wounds such as a prick from a thorn can allow enough bacteria to get into the body to cause tetanus. The illness takes up to 21 days to develop, sometimes more. Therefore, you may have forgotten about a small cut before the illness starts.
If you are to travel to areas where medical attention may not be available, then a dose of vaccine may be advised. This is even if you have had five previous injections. In particular, if it has been more than 10 years since your last injection. This is a precautionary measure in case you have a very dirty wound and do not receive antiserum. The vaccine stimulates your body to make antibodies against the tetanus toxin. These antibodies protect you from illness should you become infected with this bacteria.