Probiotics are microorganisms that are believed to provide health benefits when consumed. The term probiotic is currently used to name ingested microorganisms associated with beneficial effects to humans and animals. Probiotics are popularized in the lay literature for many different clinical problems. They have been studied in infants and children as a preventive or treatment for a variety of infections. Studies on the medical benefits of probiotics have yet to reveal a cause-effect relationship, and their medical effectiveness has yet to be conclusively proven for most of the studies conducted thus far. The putative benefit of probiotics in the prevention of infection relates to potential benefits to the innate and adaptive immune systems of infants.