Probiotics are organisms that contribute to the health of the intestinal tract. They're the beneficial bacteria that help fight illness and disease. Many doctors prescribe acidophilus in conjunction with oral antibiotic treatment because antibiotics destroy beneficial intestinal flora, often causing diarrhea as well as an overgrowth of the fungus Candida albican. This fungus can grow in the intestines, vagina, lungs, mouth(thrush), on the fingers, or under the nails. It will usually disappear after a few days' use of generous amounts of acidophilus culture.