Cellulitis is a spreading bacterial infection just below the skin surface. It is most commonly caused by Streptococcus pyogenes or Staphylococcus aureus.
The word "cellulitis" actually means "inflammation of the cells." Specifically, cellulitis refers to an infection of the tissue just below the skin surface. In humans, the skin and the tissues under the skin are the most common locations for microbial infection. Skin is the first defense against invading bacteria and other microbes. An infection can occur when this normally strong barrier is damaged due to surgery, injury, or a burn. Even something as small as a scratch or an insect bite allows bacteria to enter the skin, which may lead to an infection. Usually, the immune system kills any invading bacteria, but sometimes the bacteria are able to grow and cause an infection.
Once past the skin surface, the warmth, moisture, and nutrients allow bacteria to grow rapidly. Disease-causing bacteria release proteins called enzymes which cause tissue damage. The body's reaction to damage is inflammation which is characterized by pain, redness, heat, and swelling. This red, painful region grows bigger as the infection and resulting tissue damage spread. An untreated infection may spread to the lymphatic system (acute lymphangitis), the lymph nodes (lymphadenitis), the bloodstream (bacteremia), or into deeper tissues. Cellulitis most often occurs on the face, neck, and legs.
Belinda Rowland PhD, The Gale Group Inc., Gale, Detroit,