A friend told me about this today and I had to dig into it online because I didn't believe it, and yeah, it's completely real. When the baby is sick, the mother's milk increases white blood cells and TNFα, and the mechanism is wild: the baby's saliva enters through the nipple, the mother's body detects the pathogen, and she produces specific antibodies against that exact infection. Many mothers say the milk turns more yellow or thicker, like colostrum, while the baby is sick. Mothers' biology is insane.

Baby and breast: a dynamic interaction by Josef Neu

Backwash from nursing babies may trigger infection fighters By Laura Sanders