This all makes sense but, I also believe that if people became too hygienic, our immune systems would weaken.

If we were never exposed to bacteria on the ground but then fell and grazed ourselves, wouldn't our immune system have a harder time of fighting off infection than if it had already been exposed to floor-bacteria?

A little bacteria never hurt anyone. Hell, vaccinations expose people to all kinds of things in order to create immunity to diseases. Then again, I'm no biologist...