Experts suspect this is all based on the way the milk acts in babies' guts. The vast majority of our immune systems develop from our gut.
Now some new moms are sharing their excess breast milk with other families through milk banks. There, the milk is pasteurized, screened, tested and then sent to sick babies.
Mothers who donate must date the milk and keep track of any cold or other sicknesses in their homes. The milk collected during those days is not eligible for donatoin.
There's also growth in less official milk sharing sites. Since the informal group doesn't go through rigorous medical testing, the national site says they respect the right of families to make informed choices about risks.