It is well known that exclusive breastfeeding provides protection against diarrhoeal infection, but the mechanisms behind this protection are poorly understood. In addition, it is well known that feeding a baby artificial baby milk dramatically increases the risk of diarrhoeal illness severe enough to require medical treatment, including hospitalisation, but the mechanisms by which artificial baby milk facilitates infection are largely unrecognised. This paper uses non-technical language to describe some of the known ways in which breastmilk provides infants with external immune support. Detailed knowledge of the importance of breastfeeding and the risks associated with the use of artificial baby milk will assist more mothers to exclusively breastfeed.