There is a degree of balance between dirty and clean. People NEED to get exposed to germs to build immunity.
The classic example is polio. Researchers found when they studied it that certain “peoples” such as blacks, Latinos, and other minorities were getting polio a lot less than white people. And those minorities that got it, got it less severely. Now there are plenty of diseases which have a genetic element like this. For instance, sickle cell anemia occurs more often in African people.
But the researchers then discovered something else interesing regarding polio. The rich blacks, the rich Asians, the wealthy Latinos, and the well-off immigrants were getting polio at the same rate as whites, furthermore the poor white people were not getting polio and had the same ratio as the poor minorities.
So research started looking at “are the rich people buying or eating something, the poor people can’t afford?”
Long story short -> Turns out poor kids play in dirt. Well all kids love to play in dirt and mud, but the rich or middle class didn’t do it much and were cleaned up asap.
There is a germ that is commonly found in dirt. This germ is similar enough to polio to sort of “kick start” the immune system. Now this isn’t a case of like Cowpox providing immunity to Smallpox, what it is was that when a person is infected with polio by the time the body sees it and starts fighting it the virus is way ahead. This germ is similar to polio to activate an immune response so when polio virus invades, the body is like “Hmmmm, something’s up,” and immeidately starts to fight, so the polio doesn’t get a chance to take hold.
And that explained why these poorer kids who didn’t have exposure to clean areas to play in, or fancy playground, these kids that found their day’s entertainment in the mud, had a leg up over the fancy, clean rich kids.
So you see your immune system does need germs to keep it active. But there is a balancing act.
This is why vaccines in some cases no longer provide life long immunity in a lot of cases. In the old days, you got the mumps vaccine, let’s say. Since others had the mumps, you were constantly exposed to it, in very small does. Everytime your vaccinated body sees this tiny amount of mumps virus, it keeps up the immunity.
If 10 years go by and your vaccinated body hasn’t seen a mumps virus, 'cause everyone else is getting vaccinated too, the body says “Why should we expend energy keeping up immunity for mumps when we don’t see it. It can’t be a big threat.” Thus your immunity decreases