 We propose to follow 2,100 HIV-infected pregnant women and also some HIV-uninfected women from the time that they book at the clinic until 24 months of age. HIV-infected women who say they intend to breastfeed and all the HIV-uninfected women will be visited at their homes by breastfeeding counsellors both before and after delivery to support exclusive breastfeeding. HIV-infected women who choose not to breastfeed will be helped by clinic staff to safely replace all breastmilk with some other milk. An independent team will visit all mothers at their homes and collect information about the way they feed their children. Blood samples will be collected from the children at different times by a simple heel prick and the blood stored on a piece of filter paper. By testing these samples and comparing with the type of feeding at that time, we will be able to see when a child becomes infected and whether exclusive breastfeeding gives any protection. 