Cellulitis is a spreading bacterial infection just below the skin surface.

The word "cellulitis" actually means "inflammation of the cells." Specifically, cellulitis refers to a bacterial infection of the tissue just below the skin surface. It is most commonly caused by Streptococcus pyogenes or Staphylococcus aureus. Skin is the first defense against invading bacteria and other microbes. An infection can occur when skin is damaged due to surgery, injury, or a burn and bacteria infects the wound.

Disease-causing bacteria release proteins called enzymes that cause tissue damage. The body's reaction to this damage is inflammation, which is characterized by pain, redness, heat, and swelling. 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.

A very serious infection, called orbital cellulitis, occurs when bacteria enter and...