Our bodies contain a lot of bacteria and viruses in our digestive system. These bacteria actually help us digest food and make vitamins for us in return. They also take up space that harmful bacteria would otherwise occupy.
Inside our cells we have an organelle called a mitochondria. We use it to make energy for our cells. Some scientists believe that mitochondria are actually ancient bacteria and that cells evolved together with them, giving them shelter in return for energy. This is called endosymbiosis theory. Similarly some believe plants did the same with chloroplasts, which allow them to photosynthesise.
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