 Entry into the study requires the intent to breastfeed exclusively for a minimum of six months. If mothers decide to discontinue breastfeeding during the study, the investigators will note that in the infants chart and obtain an additional series of weekly stool samples for six weeks after discontinuation of breastfeeding. The purpose of this additional stool sample collection is to determine if discontinuation of breastfeeding has an impact on the level of existing B. infantis colonization in the infant gut. 