To understand how yogurt works, you first have to understand how the bacteria in yogurt work.
The good bacteria in yogurt are often called probiotics. They come in the form of culture concentrates in certain foods, dietary supplements and fermented dairy products, like yogurt or cheese [source: WHO]. Probiotics are usually bacteria, but yeast can act as a probiotic, too. These good bacteria are used to ferment milk. Sometimes manufacturers add other bacteria that are not considered to be probiotic.
Your gut already has thousands of types of bacteria, whose job it is to aid in digestion. The good bacteria in a healthy system stave off any of the pathogenic, or disease-causing, bacteria. Together they're called our colonizing microbes, and each of us has a unique set. When you came into this world, you had very few bacteria in your little body. Breast milk is the first, and quickest, way to transfer essential microorganisms to a newborn. Development of gut microflora begins early and the makeup changes often, but maintaining the balance of good and bad is important in developing a mature immune system.
The same sort of balancing act is also constantly happening in other parts of the body. For example, a healthy number of lactobacilli live in the vagina and inhibit harmful bacteria. If the number of lactobacilli drops, a vaginal infection can occur. Some evidence suggests that eating yogurt may be a way to maintain that positive balance.
First, the bacteria must get through all that tough-to-survive gastric acid in the stomach and into the intestine. Many national organizations set minimum standards for the numbers of bacteria in yogurt. One of these is the National Yogurt Association in the United States, which requires 100 million bacteria per gram in products that carry the Live and Active Cultures seal [source: National Yogurt Association].
The truth is that probiotics have to be introduced in those numbers to survive the digestive process. Researchers are testing what is left in the stool to determine what specific strains might get through. Some say yogurt is a good vehicle for these bacteria because the food acts as a buffer against the acid. This way the bacteria may be protected long enough to make it through the gut. But there are other factors in play, too, including the other contents in yogurt and the way it's stored.