The Gemora questions how a nursing baby can remain Tahor from a nursing mother who is Tamei. But why shouldn't he remain Tahor, ... since we have a rule that Tumah BaLuah is not Metamei and the milk is Baluah going into the baby's mouth which is also Baluah
Food or anything else that is in the mouth is not considered Tum'ah Belu'ah. See Rambam, Hilchos Tum'as Kelim 14:5.
Wishing you a Kesivah va'Chasimah Tovah,