https://www.scientificamerican.com/article/c-section-babies-are-missing-key-microbes/

How a baby is born has a profound impact on their microbiome — the community of microbes that colonize the body.

That’s the finding of the largest ever study of the newborn microbiome, which offers the strongest evidence yet that children born through the vaginal canal carry different microbes than those delivered by caesarean section. Newborns delivered by c-section, the study found, tend to lack strains of gut bacteria found in healthy children and adults. Instead, their guts harbour harmful microbes that are common in hospitals.