I'm just going to link to three articles dealing with recent studies on these matters.They are to some extent all related because of the focus on infant and childhood development of systems and processes that interact in ways we do not fully understand andpersist through our entire lifetimes.
The infant gut microbiome: New studies on its origins and how it's knocked out of balance -- ScienceDaily
Educating the immune system to prevent allergies -- ScienceDaily
Infant antibiotic use linked to adult diseases -- ScienceDaily