Though at birth the human digestive tract is nearly sterile, that of a child or adult contains a complex mixture of many different bacterial species known as the intestinal microflora. Some of the most famous thoughts in this area were developed by Elie Metchnikoff in the
early 1900's. Metchnikoff proposed consumption of fermented milk would improve intestinal health, leading to a longer, healthier life. Since that time, researchers have demonstrated that changing the intestinal microflora can affect well-being and susceptibility to certain acute and chronic diseases.
Probiotics have been studied for their ability to stimulate the immune system, to prevent diarrhea, to reduce gut colonization by pathogenic bacteria, to exhibit anticarcinogenic activity, and to reduce serum cholesterol, lactose intolerance and allergic reactions to foods. However, not all probiotics have the same potential health benefits. Probiotic effects vary from strain to strain within the same species, and not all strains have been studied to evaluate their potential health benefits.