The skin supports its own ecosystems of microorganisms, including yeasts and bacteria,
which cannot be removed by any amount of cleaning. Estimates place the number of
individual bacteria on the surface of one square inch (6.5 square cm) of human skin
at 50 million though this figure varies greatly over the average 20 feet2 (1.9 m²) of
human skin. Oily surfaces, such as the face, may contain over 500 million bacteria per
square inch (6.5 cm²). Despite these vast quantities, all of the bacteria found on the
skin's surface would fit into a volume the size of a pea. In general, the microorganisms
keep one another in check and are part of a healthy skin. When the balance is disturbed,
there may be an overgrowth and infection, such as when antibiotics kill microbes, resulting
in an overgrowth of yeast. The skin is continuous with the inner epithelial lining of the
body at the orifices, each of which supports its own complement of microbes.