The word probiotic was coined in 1965 by two microbiologists, Daniel Lilly and Rosalie Stilwell, who noticed that some microorganisms produce materials that enhance the growth of other species. Conceptually, this is the opposite of antibiotics, which produce an organism that kills or inhibits another organism. Probiotics are living cultures of bacteria and yeasts that are thought to enhance the health and well-being of patients. The bacteria incorporated in most probiotics are often the same as (or closely related to) the bacteria that convert milk into yogurt or kefir. Some are commonly found in the gastrointestinal tract of normal, healthy people. The probiotic chosen for Savino's study in Italy contains exactly this kind of bacterium, called Lactobacillus reuteri.

Historically, naturally soured milk has long been known to protect against intestinal infections. (I always assumed this was a simple matter of the acidity suppressing the growth of intestinal pathogens, but recent probiotic research hints that there's more to it than simple low pH.) The great pioneering Russian microbiologist Ilya Mechnikov argued 100 years ago that the consumption of yogurt was associated with longevity.