Is it safe to breastfeed someone else's baby?

A friend (who has a 10 month old) looked after my three month old son for a few hours. When I returned, she told me he was hungry, so she breastfed him! I was quite shocked. Is this safe?
Goodness, you must have been surprised! I am sure your friend was well meaning. But, as with any body fluid, breast milk can contain HIV, hepatitis, tuberculosis, syphilis and other viruses. They can be transmitted through breastmilk. La Leche League International advises its breastfeeding coaches about the potential problems with wet or cross-nursing. In addition to potential infection, regular cross-nursing can totally confuse and frustrate the baby, to the point that it might refuse to nurse at all. And milk from the baby's mother is exactly formulated for what her own baby needs. Another woman's milk probably won't meet the needs of the nursing child, especially if there is a big difference in age. That said, the personal nature of wet or cross-nursing can make people uncomfortable, but it is a decision as personal as breastfeeding itself.

Milk banks, which screen donors, are a possible solution for mums. At milk banks, the milk is pooled, pasteurised and checked for contamination. Many preemies and sick babies in the hospitals are receiving the milk from these banks. But, again, this milk is screened as much as donor blood would be screened for safety.

Maybe next time you leave your baby with your friend, you can leave her with your breast milk and a baby cup. That way your baby will be getting your milk without any chance of possible contamination or nipple confusion. Good luck!