Gut bacteria
The New York Times describes an experiment on how gut bacteria develop in infancy:
Seeking to understand how the microbes are developed in early life, a team of Canadian researchers collected samples (dirty diapers, frozen) from newborns at birth and again at 3 months. They found that many children who were delivered by C-section all but lacked a group of critical bacteria found in those who were delivered vaginally. The gut bacteria in children who were fed only formula, rather than breast milk, was also significantly different from those who were given at least some breast milk.