Probiotics is the technical name of those healthy, live bacteria in yogurt. Probiotics are essential for our bodies because they fight off the "bad" bacteria living in our intestinal tract.
So to keep away the bad bacteria, eat the good bacteria!
Tip: Look for yogurts that have the "contains live & active cultures" seal on the product. Don't buy yogurt if it does not have this seal: you're not getting your healthy bacteria if the stamp is absent!
Many grocery chains carry very cheap, very healthy brands of yogurt with this "live & active culture" seal. My favorite is Dannon "Light & Fit" Plain Vanilla Yogurt, which costs about $.88 at the grocery store.
Another benefit of yogurt is that it contains tons of calcium, which is great for any active person because it keeps bones strong and fends off osteoporosis later on in life. Take a Vitamin D supplement to help your body absorb all that wonderful calcium!