Specialised epidermal secretions developed as nutritious and bacteriostatic factors some 120 million years ago; milk production has proved a crucial factor to mammalian survival in a wide range of habitats. Milk composition differs considerably between phyla, within species, and with time in an individual lactating mother. The neonatal period claims the greatest infectious toll in mammals so that from an evolutionary standpoint there must be a balance in favour of producing and consuming milk without increasing susceptibility to infection. Competitive interaction with viruses, bacteria, and protozoans has resulted in the development of unique characteristics within breast epithelial cells. Unlike equivalent cells in sweat or salivary glands, they secrete nutritive molecules, antibiotic substances, growth factors, inflammatory cytokines, and chemokines while regulating a physiological recruitment of lymphoid and myeloid cells from the circulation into the milk. Milk therefore has functions other than nutrition; milk is a complex mixture of cells, membranes, and molecules. Epidemiological data from the HIV pandemic have highlighted our lack of knowledge about this secretion.

It was established in the 1960s that milk was a significant source of infection to mouse pups for Moloney leukaemia virus, sarcoma virus, and mammary tumour virus: other species show similar patterns of transmission of lentiviruses.1 In man the RNA retroviruses including HIV-1, HTLV-1, and HTLV-2 are all transmitted by this route.2 It has been recorded that HIV-2 is not transmitted by breast milk, but it is probable that there is a relatively lower risk in this less virulent retrovirus as well as fewer data to assess infectivity. Cytomegalovirus is possibly the most commonly detectable virus in milk: it is thought that reactivation of virally infected breast epithelial cells in early lactation promotes the shedding of infectious free virus particles.3 Rubella, herpes simplex, and rarely hepatitis B can be passed on …