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I have always been comfortable with the concept of there being bacteria and such living in our intestines, hair, and everywhere else in our body.But thinking of the actual individual cells that make up our body as another set of single celled creatures that live inside us, not necessarily because of us, seems to be something completely different altogether. Although there is an interdependent relationship between the unicellular and the multicellular within the multicellular organism, why is it that these living cells work together forming a respiratory system for the rest of the body? Bacteria and viruses live within us to survive, why aren't the rest of them just as self-concerned? How did this come about?