[schu]

The main factor at play here is maternal passive immunity: https://en.wikipedia.org/wiki/Passive_immunity#Maternal_pass...

As far as I know, there's no (direct, retrograde) transfer of pathogens from the child to the mother through the breast. But if the child has an infection, there is often a higher chance for the mother to get ill as well (due to close body contact, aerosol particles, etc.) and to develop a "mature immune response", which could lead to a (delayed) positive secondary effect on the child through passive immunity.