In the Marvel comics universe, an alien race known as the Symbiotes have repeatedly plagued the citizens of Earth, particularly the superhero Spider-Man. He encountered the first Symbiote when both of them were transported together by a cosmic being -- Spider-Man thought it was simply a black costume made of alien material before realizing it was a living creature. The Symbiote later transferred to mentally disturbed reporter Eddie Brock, forming the monstrous creature known as Venom. Reproducing asexually via budding, the Symbiote’s first offspring bonded with a serial killer and became the creature known as Carnage.
It was eventually revealed that the Symbiote race was parasitic, invading entire planets, bonding to whatever living beings they found and consuming them. They may be responsible for the extinction of tens of thousands of species as they traveled from planet to planet across the galaxy. Spider-Man’s Symbiote was an insane outcast of its parasitic race.
Are you a symbiote?
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.