Breastmilk can change when your baby is sick or exposed to illness. In fact, researchers believe that during breastfeeding, baby’s saliva reacts with breastmilk to produce reactive molecules. When a babies are sick, they pass on a cue through the saliva that sends a signal to the mother’s body to produce more milk with illness-specific antibodies. Similarly, if the breastfeeding mother is exposed to a virus, she will produce antibodies that are passed through her milk to the baby for protection.
A study done in 2013 found that when mothers and babies both had colds, levels of white blood cells in milk jumped by a factor of 64. But even when just the babies were sick, levels of white blood cells in breast milk still increased by 13-fold. Suggesting that there is a co-interaction between the mothers and the baby’s immune system that is mediated by breast milk.