I think the problem is that there is a ring of truth to that (Vaccines work off the same principle: give your body a weakened form of the virus to practice on, so when the real thing comes along it's more prepared), but it's only true to an extent. Purposely exposing yourself to potentially lethal diseases like polio, measels, etc. is stupid and dangerous. Rolling around in the dirt might make you more resistant to minor infections and even some viruses, but it's not going to be as effective as vaccines.

Think about it. Smallpox killed millions of people for thousands of generations, even though most of those people were unvaccinated and were dirty pretty much all the time. It took a vaccine to end smallpox once and for all. I mean, do you realize how incredibly fucking lucky we are to have been born during a time when smallpox is all but eradicated? The vast majority of human history has been one where smallpox killed people left and right. And we just happened to have been born during the tiny sliver of human history when it only exists in a lab somewhere.

People look to the time before basic hygiene as if it were some kind of golden age where everything was great. Everything was not great. Babies died all the time from unhygienic practices (doctors not washing their hands after touching dead bodies because they didn't know about germs) in hospitals. Women died in childbirth quite often. People would have lots of kids because they'd figure a few of them would die (seriously, I read my great-great-great grandmother's obituary and it made a point to say that she was lucky to have all of her children alive when she died).

Sure, there are some downsides to living in a more sterile society, but those downsides pale in comparison to the health situation that existed 100 years ago.