Are you a symbiote? Absolutely. Your digestive tract contains trillions of bacteria and other microorganisms. In fact, most of the mass of fecal matter is made up of bacteria. These bacteria serve a number of functions, but they primarily break down things that our digestive system is unable to process by itself. For example, a lot of carbohydrates make their way to the intestines undigested. The bacteria there break the carbs down into various acids that can be absorbed and processed. The result: We get more nutrients and calories from our food. Antibiotics can kill off a lot of these bacteria, reducing our digestive efficiency until they grow back [source: University of Glasgow]. The bacteria, for their part, get a steady supply of food delivered straight to them.
This digestive aid is a great benefit to people with limited access to food resources. They need to get every calorie they can from their food. However, scientists have been studying the contributions of human gut bacteria to widespread obesity in Western nations. Experiments have shown that mice raised in a sterile environment, with no bacteria to aid digestion, remained lean even though they were fed a high-calorie, high-fat diet [source: PNAS]. Manipulating our own symbiotic relationship with gut bacteria could lead to the development of an effective diet pill.
The bacteria in your gut are pretty complex, providing benefits we don't fully understand yet. Some scientists think they might aid our immune system by providing "practice," allowing us to produce antibodies that protect us against more harmful microbes. Indeed, gut bacteria themselves can be very harmful to us if they move out of the digestive tract into the bloodstream. They might also outcompete microbes that would be harmful if they were able to move in and live in our intestines.