Concerns with nursing another's baby
My question is, what are the possible infections that could be present in breastmilk if a mother other than the birth mother breastfed the baby?
I am not sure by your question if you are thinking of nursing someone's baby, or thinking of asking another mother to nurse yours, in your absence. If either or these are the case, this is something that should not be done without a lot of careful deliberation. Though the possibility of disease transmission is a major issue, it is not the only one to consider.
The breastfeeding relationship is a very intimate one, not unlike the relationship between a husband and wife. I am very aware that cross-nursing has a long history, and is still practiced more than most of us could imagine. Throughout the world cross-nursing is thought to create a very special bond, and children who are nursed by the same women are considered to be "milk brothers" or "milk sisters". Some people consider nursing another's baby to be breaking the intimate bond between the two. (Similar to your husband sleeping with another woman in your absence.)
As far as diseases, transmission of hepatitis and HIV are of concern when breastfeeding another woman's baby. It is impossible to be aware of the health status of every sexual partner a woman (and her partner) may have had. Cindy Turner-Maffei, MA, IBCLC, states, "I would be unwilling to allow another woman to nurse my child, without knowing the health status of every sexual partner of the woman's recent sexual partner(s). Our personal communities are much larger (and paradoxically, much smaller) now than ever before. People can circle the globe in days, allowing viruses and bacteria to spread like never before. Many people have multiple sexual partners. A small traditional community (where diet and health status are quite similar) is a very different thing from the average community of today. The only kind of cross-nursing I believe safe to recommend today is the use of donated human milk from a certified milk bank."
If you do decide to cross-nurse, it is important that all parties involved be made aware of the possible health risks, and also the emotional sequelae that may result. Very best wishes!