Snapshot An 18-year-old man presents with a painful rash on his upper arm. He reports a recent mosquito bite in the same area three weeks ago. Since then, the area has become red, painful, and hot. On physical exam, his physician notes a well-demarcated raised area that is bright pink and hot to the touch. He is sent home on penicillin. Introduction Clinical definition bacterial infection involving the upper dermis and cutaneous lymphatics often from S. pyogenes from superficial involvement of skin to deep impetigo (very superficial skin infection) erysipelas (upper dermis and cutaneous lymphatics) cellulitis (deeper dermis and subcutaneous tissues) Epidemiology demographics common in infants, children, and older adults risk factors existing skin injury lymphedema venous insufficiency diabetes hypertension prior streptococcal throat infection tinea pedis Etiology S. pyogenes most common S. aureus Pathogenesis pre-existing injury in skin can act as entry portal for bacteria Prognosis recurs in 10-40% of cases Presentation Symptoms painful and tense skin Physical exam well-defined bright pink and erythematous plaque raised above level of surrounding normal tissue clear line between involved and uninvolved tissue warm Studies Labs none needed Diagnosed clinically Differential Cellulitis deeper involvement of skin and soft tissues there is often an overlap and may not be distinguished clinically from cellulitis Treatment Conservative elevate affected area indications for all patients Medical penicillins indications for mild infection cephalosporins indications if there is contraindication to penicillin Complications Sepsis