system
Given the following post, respond as if you're commenting on Reddit:
human
How is the human gut microbiome established in infancy or earlier on?

Since we have a whole plethora of microorganisms living in our gut system, I was wondering how early/how that is developed in humans? Does it develop while we’re in the womb and if so, how? I hope this question makes sense and thanks in advance!!
gpt
Put simply, babies are most often born in blood and shit. It's messy. And that shit (or thin liquid, or mucus) contains gut bacteria which the baby needs to survive. All it takes is brief contact. Even a birth in water is not free from this issue, the bacteria get into the water. Gut bacteria are crawling all over the anal/genital region all the time, as well.

Babies born under overly-sterile conditions often suffer digestive problems; that messy birth is actually helpful.

They also get some gut bacteria from the mother's mouth, breasts, and any other part of the body they suck or lick.

Fecal bacteria - gut bacteria - get all over everything in such situations, and some of it gets into the babies mouth. Welcome to biological life; it's disgusting, but it works.