People need germs to stay healthy.

Gut bacteria helps us digest food, but we often regard the germs outside our bodies as vile, filthy things. While some of them are, we need these germs to stay healthy. 

The human immune system adapts to different attackers, which is why our species has survived for so long. When our bodies come into contact with a foreign substance, the immune system attacks it and retains this information. If this particular foreign substance comes back, the immune system knows what to do. This action can be seen most commonly with measles: a single infection is usually enough to protect a person for life.