Cellulitis is an acute bacterial infection of the dermis and subcutaneous tissue, most often caused by Staphylococcus aureus or Streptococcus pyogenes. It usually begins at a site of minor trauma and spreads rapidly; systemic involvement may follow if treatment is delayed.

1. Local skin changes
   Erythema expands with irregular, slightly raised borders. The area feels warm and tight, and surrounding skin may take on a glossy, stretched appearance.
2. Pain and tenderness
   A steady, throbbing ache begins soon after redness appears. Even light touch or clothing pressure becomes uncomfortable; tenderness often extends slightly beyond the visible erythema.
3. Swelling and induration
   Oedema develops as capillary permeability increases, causing pitting or brawny induration. When the lower limb is involved, ankle or foot swelling can make walking difficult.
4. Systemic response
   Fever, chills, and malaise commonly accompany spreading infection. Lymph nodes draining the region become enlarged and tender; rigors suggest bacteraemia.
5. Blisters and petechiae
   Severe cellulitis may form bullae filled with clear or sero-sanguinous fluid, or show tiny petechiae and purple patches that indicate vascular damage.
6. Alarm features
   Rapid progression despite antibiotics, dusky or anaesthetic skin, crepitus, or severe pain out of proportion to appearance signals possible necrotising fasciitis and demands emergency care.

Prompt antibiotic therapy prevents ascending lymphatic spread and complications such as abscess, osteomyelitis or sepsis.