Supposedly the tetanus bacterium is everywhere. I think the real reason tetanus is rare is that people get scratched and cut so often that nature naturally inoculates us. If you go live in a padded room for a decade or two and then step on a rusty nail, you might be in trouble. But if you work outside and get cut and scratched a couple times per year, you are used to it and should have a healthy immune system to fight tetanus.