According to the legend a Nomad was camping on the blistering slopes of a mountain somewhere in the Middle East. The nomad forgot some milk in a knapsack. The heat and an airborne lactic acid-producing bacteria contrived to produce the very first yogurt. The Nomad was delighted with the results and kept up the production from one generation to the next. He realized that when the milk became acid it lasted longer (the production of lactic acid from lactose has a preservative effect on milk and it inhibits putrefaction).
But the wonders of yogurt didn't come to public attention until much later, in the 20th century. A Dr. Metchnikoff, who also happened to win the Nobel Prize in 1908 for his studies of the immune system, noticed that Bulgarians lived longed and better and that they consumed large amounts of yogurt. Dr. Metchnikoff saw the connection and went on to demonstrate how healthy bacteria in yogurt helped normalize digestion and improved the immune system.
Beware of commercially produced yogurt, where cutting-costs-at-all-costs is the goal. Health-wise and taste-wise there's no substitute for the real thing. The best yogurt will be made with real cultures of Lactobacillus bulgaris and Streptococcus thermophilus. It should be naturally creamy, not slimy or "gelatin-like", like some of those cheap yogurts with thickeners or coagulating agents.
Another suggestion: fruit added to yogurt is delicious, but it's best to add it right before you eat it.