Erysipelas and cellulitis are mostly caused by beta-hemolytic streptococci and S. aureus. An increasing number of cases are now caused by methicillin-resistant strains of S. aureus (MRSA), which are acquired outside of the healthcare setting.

Haemophilus influenzae and other respiratory pathogens may cause infection involving the face as a complication of upper respiratory tract infection.

Other pathogens are usually associated with certain specific exposures, such as Pasteurella following a cat bite, Erysipelothrix following a fish scale-induced injury, or Aeromonas following a fresh water traumatic injury.

Necrotizing fasciitis involving the extremities is usually secondary to S. pyogenes but may also be caused by group C or G streptococci, primarily in the elderly.

Cases of necrotizing fasciitis due to Vibrio vulnificus are reported in mostly patients with chronic liver disease with acquisition through salt water exposure.

Increasingly cases of necrotizing fasciitis have been seen due to the USA 300 strain of MRSA.

Necrotizing fasciitis due to synergy between multiple pathogens and involving either a foot ulcer or the perineum, so-called Fournier gangrene, is most commonly seen in patients with poorly controlled diabetes mellitus.

Clostridium perfringens is associated with necrotizing cellulitis or myonecrosis in patients who have had wounds contaminated by soil or compromised by the presence of devitalized tissue.

Erysipelas and cellulitis risks include being overweight, having chronic edema or chronic skin breakdown, and having a prior history of erysipelas or cellulitis.

Necrotizing fasciitis risks include diabetes mellitus and alcoholic liver disease but also may occur in a normal host.

Erysipelas and cellulitis present with erythema, warmth, tenderness, and varying degrees of edema of the skin and soft tissues. Both are typically associated with fever and chills. Erysipelas produces raised skin with a well-demarcated edge and typically begins with sudden onset and progresses rapidly over hours. Cellulitis begins more gradually, progresses more slowly often over days, and usually has less distinct skin margins. Both have a predilection for the lower legs but may be seen elsewhere depending on the circumstances. Women with chronic arm lymphedema following lymph node dissection for breast cancer may develop cellulitis in the involved arm. Patients who sustain a puncture wound to the hand due to a cat bite or to the antecubital fossa due to intravenous drug use may also develop cellulitis of the involved arm. Cellulitis may involve the abdominal wall in morbidly obese patients without any obvious trauma. Cellulitis may involve the head and neck region as a complication of upper respiratory tract or oral cavity infections.