Before it’s born, an infant’s intestinal tract is sterile. At birth, it gets its first exposure to bacteria, which begin to establish the child’s gut microbiome. These microbial populations play an important role in myriad facets of health—immunity, digestion and obesity among them. During a normal delivery, the baby is exposed to the intestinal and vaginal gut flora of the mother. In a caesarian section, the baby instead picks up its first bacteria from the skin at the incision site. This leads to the growth of somewhat different microflora, identifiable months or years later, which may contribute to higher rates of asthma among C-section babies and other effects.