Four years ago (2012) Jonathan Eisen gave a TED Talk focusing on the cloud of microbes living on and in each of us. During this talk he introduces a notion that a growing number of scientists are getting ready to maybe perhaps embrace. This notion being that this microbe cloud should be regarded as an organ.
This organ is composed of thousands and thousands of different species of microbes. In terms of size, if you add up all these individual little microbes they make a lump of icky squirmy cells forming a mass larger than the brain.
That is a significant contribution. Seems like we should be more aware of it and maybe even take a few steps to keep it healthy and in proper function.
Give this TED Talk a view for a good beginning introduction to the concepts.