How do you compare "strength" of immune system vs. higher exposure?
The immune system builds antibodies or "resistance to" things it's exposed to. If one is in a sterile environment regularly, that won't happen. Then when something does come along, they can't fight it.
Exposing sick kids to each other, or letting them play in the dirt to help build their immune systems has been common practice for a very long time. Not saying it makes people be able to fend off anything but it certainly helps with common colds, milder types of infections, etc. This doesn't end at adulthood, our systems always need to be maintained. The body will stop making what it figures it doesn't need, or what it is getting from an external source.
Sidebar...but the "external source" thing is part of how addictions work. When you get the chemical that makes the brain release dopamine or whatever from an external source, the system will stop doing that on its own...then you are dependent on the external source to function.