even ultra clean "normal environments aren't actually all that clean. If you have been raised or spent long time in a really ultra clean environment such as is used for children with impaired immune systems sometimes then yes, you are more likely to catch stuff when out of it, but for normal folk living normal lives, the immune system gets enough challenges.
Having said that, the kicker comes when you go among strangers. All of us live in happy parnership with all manner of bugs both on the skin and within. We share these with the people we live closely with, all develop a tolerance of their little ways and by and large are not damaged by them. If we then mix with strangers eg go to school, college, take on a new job in a busy enclosed environment, we meet and share new bugs and have to go through the process again. This often means having a spate of mild colds, tummy bugs etc until the new tolerance is developed. AND of course while we are doing this we are generously sharing those new bugs with our family members too.
Eating is one way to intake bugs but its not the only way. inhale over a flushing toilet that has the lid up!!, share stuff like biros and scratch your nose or lips and that old favourite, failing to catch a cough or sneeze.
Sorry if TMI but you did ask!!