Yogurt is a cultured or fermented milk product that is soured and thickened by adding specific lactic acid-producing cultures to milk. The basic cultures or probiotics used to make yogurt are Lactobacillus bulgaricus and Streptococcus thermophilus. Additional probiotics are often added. Common ones are Lactobacillus acidophilus, Lactobacillus casei, and Bifidus, all of which may help to maintain the balance of bacteria needed to boost the immune system and promote a healthy digestive tract.