"The introduction of such a variety of bacteria offers the burgeoning immune system of the infant to decide which germs are friendly and which are foes. This form of immunological training may then help to prevent problems in the future, including overreactions such as asthma and other chronic diseases.
What was perhaps most interesting was that in healthy children, by the time they reached four years of age, they had the same diversity as their mothers before pregnancy. This cyclical nature of microbiome diversity suggested that for best health, infants needed to be exposed to a large variety of germs."
Jason Tetro: Germs May Hold the Key to Infant Health