Oral supplements of live, beneficial intestinal microorganisms for nutritional health and well being are known as probiotics. Bacteria are continually in competition for colonization space. The original bacterial colonies have an advantage over transient bacteria. Nevertheless, the composition of the intestinal microflora is dynamic and constantly changing. If harmful organisms proliferate, the equilibrium is disturbed, and it becomes difficult even for indigenous bacteria to maintain their necessary territory to colonize the intestine. Antibiotics tend to kill off both beneficial and harmful bacteria, and thus may disturb the normal, healthy balance of intestinal microorganisms. Saccharomyces boulardii is a nonpathogenic yeast that can support the gastrointestinal tract. Research studies have indicated that S. boulardii may work by modulating the body’s immune response, thus helping to improve the response to undesirable microorganisms.