Cellulitis is an infection of the skin and deep underlying tissues. Group A strep (streptococcal) bacteria are the most common cause. The bacteria enter your body when you get an injury such as a bruise, burn, surgical cut, or wound.
Your health care provider may take a sample or culture from your skin or do a blood test to identify the bacteria causing infection. Treatment is with antibiotics. They may be oral in mild cases, or intravenous (through the vein) for more severe cases.
NIH: National Institute of Allergy and Infectious Diseases