Chewing food for your baby.

I'm sure it's more common than you think. There are societies in which pre-chewing baby food is the norm.
It's not all the bacteria in the mouth, (babies need some anyway) but plaque. Plaque is like an infection, once you've got it, you'll always have it. Babies don't have it when they're born but will eventually catch it.
My dentist wanted me to start taking bliss tablets (like inner health plus for your mouth) so that baby would get the good bacteria from me more than the bad.