The average adult human body has about 1013 cells, around 1600 times greater than the number of human beings on Earth. It is estimated that for every cell in the human body, there are about ten microorganisms, mostly bacteria in the large intestine. This is possible because the volume of a typical bacteria cell may be more than 1000 times less than one of the body's cells.
Along with the bacterial body flora, everyone is also colonized by fungi (mainly yeasts), protists, and archaea (mainly methanogens), although less is known about these because of their scarcity relative to bacteria. In the domain of macroscopic body flora, when found inside the body, these are usually harmful parasites such as tapeworms. However, certain humans may have thousands of macroscopic body flora in their skin or hair, often in the form of mites. Scientists believe there may be as many as a million mite species in the world, adapted to every conceivable environment -- including the human body.
99% of bacteria in the gut comes from 30-40 species. Commonly sighted genera include Bacteroides, Clostridium, Fusobacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus, and Bifidobacterium. Bacterial body flora tend to have a symbiotic relationship with their host. Bacteria help digest complex carbohydrates which would be indigestible otherwise, promote growth of intestinal cells, repress pathogenic microbes, prevent allergies, inflammatory bowel disease, and play crucial roles in the immune system. Body flora and the body it occupies have been co-evolving for tens of millions of years.
About 60% of the mass of feces is made up of bacteria. Some bacteria found in the feces can be pathogenic to the person it came from and to people around them. This is part of the reason for modern hygiene. Bacteria spread via feces include E. coli and Vibrio cholerae, the bacterium that causes cholera.
