Children raised in a sterile environment fail to become inoculated early on, later on they come in contact with a near "benign" germ and their body can not handle it. The process of growing up on planet earth means that one must be inoculated against millions of bacteria and virus strains - each do, over time, build up your resistance to other "stronger" relatives of X strain. The yearly cold/flu is a nuisance to healthy people, however it is a fire drill, a test run for the body which teaches the immune system how to mount a defense and make war with viruses/bacteria. Thus an otherwise "healthy" person who says "I never had a cold in my life" can be knocked flat with the first cold they get - which could (and often does) complicate into bronchitis or what used to be called "walking pneumonia". A weakened body, one that rarely has practice runs will have more complications with "minor" infections than a person who's immune system gets a regular workout, test run sort of thing. so yeah sure, we might be having fewer colds and flus by keeping the house sterile, however when we do get them, we get them hard and we set ourselves up for the complications.