Fairly scrupulous with ours until weaning time.
Teething time is when immune system developing. Pretty obvious as they’ll be going from boob to food which is non sterile unlike milk.
Was on an oral medicine course a while ago (I’m a dentist) and he talked a lot about the increase in allergies. Explained it by saying that as immune system becomes slightly redundant due to more sterile environment it is more likely to react to something else eg peanuts, cat fur etc you are not born allergic you are exposed to something, your body decides it doesn’t like it then next exposure the immune system reacts against it.
He finished his lecture by advising we let our kids play in the mud and buy them a dog!