"But of all the places available to our normal flora, the gut is the most heavily occupied. Why? Because the human digestive tract offers the microbe what New Zealand offers the tourist: a massive variety of environments creating opportunities for a plethora of activities. It is a multicultural melting pot. With something for almost everyone, it's no surprise that the human gut hosts 80% of the body's normal flora and includes more than 500 different bug species."
Gut Flora: A Digestible Account of Probiotics - The Naked Scientists