My daughter cut her hand on a piece of glass, so I took her to the doctor's. It was a bad enough cut that they gave her an anti-tetanus serum.
I was curious, so I asked the nurse what actually causes tetanus. She told me that it comes from a bacteria infection that is found in the dirt.It comes off of animal feces as spores with bacteria inside.
When you get a puncture from a rusty nail or other dirty thing, it's probably in the dirt, and you might get some of the bacteria into your body.
Then the bacteria grows and makes a toxin. This toxin can cause muscles to tighten up. When this happens, it might make your jaw muscle lock and you can't breathe This is such a serious disease, that you can often get the booster shot for free, so everyone will have it.