Saccharomyces boulardii is a tropical yeast that grows on the skins of tropical fruits like lychees and mangosteens. People in Indochina have been using it to treat digestive problems for centuries. Thanks to Dr. Henri Boulard, who brought this strain to France nearly a century ago, and Biocodex, a French natural medicine company, S. boulardii is now among the most widely used probiotics in the world.