Your body is host to about 100 trillion microbes, most in your colon, where they live and grow, aid in digestion, and prime the immune system. More than 400 different types of bacteria live in the gastrointestinal tract alone. Most of these bacteria are not harmful, and some are actually beneficial and important for normal growth and development. Under normal circumstances, the “good” bacteria far outnumber the bad, but any shift in the balance of bacteria may affect how well your gastrointestinal tract functions.