A few days ago I had a conversation with someone (don't remember who) about how we felt like working with kids at our service-learning placements had improved our immune systems. I've personally interacted with a fair number of runny-nosed students and my roommate was sick last week, but I haven't gotten sick, and normally I catch colds pretty easily. I've heard a little about how exposing young children to less-sterile play environments can boost their immune system function later in life and wondered if something similar was happening here, so I decided to look into it.
Turns out I was misinformed—exposure to pathogens and microbes doesn't provide an appreciable boost in our immune systems' ability to fight off sickness faster, and it doesn't seem to help us avoid getting sick either. What it does do is help regulate our immune systems' first-line response: inflammation. Exposure to germs and viruses as a child can improve our immune systems' capacity to handle allergens, asthma and other inflammation-causers, lowering the amount of inflammation we experience and many of the detrimental side effects it can cause to our bodies.
Studies have shown that only bugs associated with "dirtiness" end up helping improve our immune systems—respiratory infections seem not to have any positive effect. exposure to fecal matter in particular seems to lower levels of c-reactive protein, which is a key component in human inflammatory response.
one idea that can help explain this positive effect is that humans, in some cases, may have evolved in response to colonization by bacteria and viruses. the selective advantage of inflammation repression conferred by pathogen presence may have helped some of our ancestors survive long enough to reproduce while those whose bodies didn't experience reduced inflammation may have died sooner, failing to reproduce.
more information here, it's super cool!: