“For an immune system to work properly, it needs to be confronted by an infection in the first year of life,” says Greaves. Without that confrontation with an infection, the system is left unprimed and will not work properly.”
And this issue is becoming an increasingly worrying problem. Parents, for laudable reasons, are raising children in homes where antiseptic wipes, antibacterial soaps and disinfected floorwashes are the norm. Dirt is banished for the good of the household.
In addition, there is less breast feeding of infants and a tendency for them to have fewer social contacts with other children. Both trends reduce babies’ contact with germs. This has benefits – but also comes with side effects. Because young children are not being exposed to bugs and infections as they once were, their immune systems are not being properly primed.
“When such a baby is eventually exposed to common infections, his or her unprimed immune system reacts in a grossly abnormal way,” says Greaves. “It over-reacts and triggers chronic inflammation.”
From https://www.theguardian.com/science/2018/dec/30/children-leukaemia-mel-greaves-microbes-protection-against-disease, about how childhood leukemia develops. Also impacts Type I diabetes, allergies, and other auto-immune disorders.