Acidophilus is a nutritional supplement product, often added to milk or sold as a capsule, which contains one or more of the following bacteria which aid in digestion: -
Acidophilus contains bacteria that have a symbiotic, or mutually beneficial, relationship with the human stomach. It helps break down complex food molecules and inhibits the growth of harmful bacteria. It is especially useful for people with bacterial or yeast infections or digestive problems, as well as people on antibiotics.
Products of this type are generally referred to as probiotics.
Eli Metchnikoff won a Nobel Prize in 1908 for demonstrating an apparent link between Lactic Acid Bacteria (the ones included in Acidophilus) and extended lifespan.