Tetanus is caused by a bacterial infection. A toxin from the infection affects the nervous system. It can lead to severe muscle spasms. Such spasms lead to lockjaw. This spasm makes it impossible to open or close the mouth. Tetanus can be fatal.

Nervous System
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Tetanus bacteria are found in soil, dust, or manure. It enters your body through a break in the skin.

When it is in your body, the bacteria create a toxin. This toxin causes tetanus.