Cellulitis on a 47-year-old woman's leg, one week after an insect bite. Cellulitis is a bacterial infection of the skin and its underlying tissues. Cellulitis is often due to infection by Streptococcus bacteria entering through a wound or break in the skin. Cellulitis may cause a fever and chills, and if left untreated, may result in septicaemia (blood poisoning) or gangrene. It is usually treated with penicillin or erythromycin antibiotic drugs.