When I think about things an infant could be exposed to from the mother I think about certain microorganisms from food is one of the first things that comes to mind. For example it is not recommended that pregnant mothers consume raw fish or sushi because the bacteria poses a greater risk to the fetus as they do not have an immune system to protect them from microorganisms. Mothers could also expose their infants to harmful substances such as alcohol and drugs if they are consuming any of that while breastfeeding which is dangerous. Infants could also be exposed to pathogens if the mother is sick through breast milk. Beneficial things that the fetus gets exposed to during pregnancy is the mothers IgG antibodies that are able to cross the placenta. This is beneficial because it helps infants build their immune system and have some sort of immunity when they first enter the world. IgA antibodies are also shared from the mother via breast milk. However, infants can also expose mothers to pathogens through their saliva when breastfeeding, so if an infant does get sick, the mother is also likely to get sick. However I think infants can also share their antibodies via their saliva to aid mothers? (I’m not entirely sure).