The relationship established between two organisms that need each other to survive is called symbiosis. Bacteria have a long history of symbiotic interactions with humans, and they have even evolved in symbiosis with other microbes and with their hosts. In fact, humans have trillions of bacteria living in their digestive tracts, and these bacteria have found a suitable ecosystem for their development. As they break down food that humans can’t digest by themselves, they produce energy and the vitamins we need.