For as long as there have been humans, they have been colonized by microbes. By far, the most numerous of these microbes are the bacteria. The number of bacteria in your microbiota outnumbers the cells in your body by about 10 fold. Because they are the most predominant among the microbes, the bacteria in the microbiota have been the topic of the most studies. There have been major initiatives internationally and in the US that catalog the bacteria that live inside, and on the surface of volunteers.
These large scale scientific initiatives have identified many correlations between a large number of human diseases and variations in the human microbiota.