On: Adult immune systems ‘remember’ germs to which they’ve never been exposed (2013)

[BurningFrog]

Those are (mostly) good bacteria though.

Some theorize the baby needs the mother's poop, to start it's own digestive microbiome. It works that way in other species, but I don't know if it's more than speculation for humans.

You also need a skin microbiome, but you can get that later.

[jacobush → BurningFrog]

I have heard reasoning that c-section babies could benefit from having some given to them.