Cellulitis of a 77 year old woman's arm. Cellulitis is a bacterial infection of the skin and its underlying tissues. It is often due to infection by Streptococcus sp. bacteria that enter 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.