I always thought (and I could be completely wrong here) that having a good natural immune system comes from lots of frequent small exposures to common infections, like colds, stomach bugs, etc. So when you encounter those things you're able to easily fight them off, because your body recognises them and has built up a good defence.
But that doesn't mean that your body is prepared to fight off something totally new and different like tetanus or polio, so we need to give a vaccine to give the body a heads up for if those pathogens ever are encountered.
Does anyone know if that's actually right though?
Haha and hygiene is kind of a balance, can't be too clean or you'll never build up natural immunity, but need to be clean enough to not catch the really bad stuff?