It has been documented/demonstrated that the commensal bacteria in the vagina are the first thing that a naturally-born baby is exposed to, and that without this exposure there is a significant delay in the development and expansion (both in number and diversity) of the commensal bacteria within the child (this process takes about 3 years to reach completion). Interestingly, cesarean babies can be smeared with vaginal secretions to simulate the vaginal birth and reverse the damage.