Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. One of the most common indications for probiotic treatment is the prevention of antibiotic-associated diarrhea (AAD). Unfortunately, many probiotic products used for AAD are not supported by rigorous independent research, and often results in non-evidence-based usage. The overarching objective is to move research forward for the most well-studied Bifidobacterium strain. The primary aim is to test the efficacy of high dose, BB-12-supplemented yogurt in preventing AAD, compared to yogurt without BB-12, in children receiving antibiotics. Other aims are to further assess the safety of yogurt supplemented with BB-12, and to carry out longitudinal community structure and gene expression analysis of fecal microbiota to evaluate the impact of high dose BB-12 in a pediatric population receiving antibiotics. The microbiota includes hundreds of species, and its disruption is hypothesized to be an important factor in the development of AAD.