This is a big oversimplification, but the bacteria that causes tetanus basically lives in the dirt. If dirt can get to it, it can have tetanus on it. And I don’t know many dogs that are above picking things up with their mouth from the ground.

There’s of course the old myth that it’s caused by rust, rust really has nothing to do with it, it just happens that if you leave something made of iron/steel outside, it tends to rust and also get dirty.

The bacteria also lives in the digestive systems of a lot of animals, so if something might have pooped on it, there’s another way for you to be exposed to tetanus, and again a lot of dogs are willing, even eager, to eat poop.And of course there’s no shortage of people and sources that are happy to tell you that basically everything in the world has a bit of poop on it in some form or another.

Also, remember that part about tetanus living in animals’ digestive systems? I hope so, it was only one paragraph ago. That includes humans, there’s a pretty good chance you have tetanus already living in your gut. In your digestive tract it’s not an issue, maybe even beneficial (we still have a lot to learn about our gut microbiomes) it’s only really an issue if it makes its way into your bloodstream/lymphatic system, which it normally can’t do except through a wound.

And deep puncture wounds, like from a dirty needle, rusty nail, or dog’s canine tooth, are kind of the ideal place for tetanus to do its thing, like most bacteria it likes things warm and moist, and your body checks those boxes nicely, and it likes a low-oxygen environment and there’s not a whole lot of airflow at the bottom of a puncture wound.