Milk mysteries: why are women who exclusively breast-feed less likely to transmit HIV during breast-feeding

(See the article by Lunney et al, on pages 762–769.) At first it seemed counterintuitive. Why would human immunodeficiency virus (HIV)-infected women who exclusively breast-feed their infants be less likely to transmit HIV postnatally than women who breast-feed but also provide their in-fants other liquids and/or solids? Aren’t the exclusively breast-fed infants ingesting more HIV-containing human milk? Yet, following the original observation from Durban in South Africa [1], at least 3 other large epidemiologic studies, 2 of them set up specifically to examine this hypothesis, also observed significantly lower rates of postnatal HIV transmission among exclu-sively breast-fed infants, compared with predominantly or partially breast-fed in-fants [2–4]. Possible explanations for the mechanisms underlying this association have been discussed [5], but there are few data examining the pathways. In this issue of the Journal, Lunney et al [6] take on the daunting task of testing one of the potential explanations for the HIV transmission–reducing effects of ex