How many species live on or in the human body?

The short answer is about 200, including 80 in the mouth alone. The total number of bacteria excreted by the body every day ranges from 100 billion to 100 trillion. Every square centimetre of human bowel is home to around 10 billion microbes. Some 10 million organisms occupy every square centimetre of flesh. The most densely populated areas of the human body are the teeth, throat and alimentary tract, where the concentrations are increased by a thousand compared to bare skin. But while such figures seem huge, it has been calculated that the amount of bacteria on the skin of the average human would be the same size as a pea.