Cellulitis is an acute inflammation of the deeper layers of the skin caused by a bacterial infection.

The skin is composed of three layers:

• Epidermis. Protective outer layer of the skin.
  
  
• Dermis. The middle layer of the skin, below the epidermis and above the subcutaneous tissue. It is composed of connective tissue which contains hair follicles, sweat glands, blood vessels and nerve fibers.
  
  
• Subcutaneous tissue. The deepest layer of skin, under the epidermis and dermis. Subcutaneous tissue is composed of fat, tissue that separates the fat in pockets, nerves and blood vessels.

Bacteria may gain access to the dermis via an external route or via a blood-borne route.

Under normal circumstances, the skin has many different types of bacteria living on it. In most cases, the epidermis prevents bacteria from getting inside the body. If the skin is broken from a cut or other means, bacteria have the opportunity to enter and grow in the body, causing infection and inflammation.

Cellulitis occurs when the deep dermis and subcutaneous tissue of the skin become infected. It can occur on any area of the body. However, children usually develop cellulitis on the face and neck, and adults develop it more often on their extremities.  

When infected, the skin tissues become red, hot, irritated and painful. The infection may only impact the skin, or it may affect other tissues and spread to the lymph nodes and blood. Fluid-filled blisters sometimes develop.

Cellulitis is different from impetigo, a bacterial skin infection generally limited to the epidermis.

In healthy people, cellulitis rarely poses serious problems. However, if untreated or not treated properly, it can lead to complications, including:

• Gangrene (tissue death) of affected areas.
  
  
• Infections and shock (a life-threatening condition characterized by the body’s inability to supply enough oxygen to meet tissue requirements).
  
  
• Meningitis (inflammation of the membrane surrounding the brain and spinal cord) may develop if cellulitis occurs on the face.
  
  
• Lymphangitis (inflammation of the lymph vessels).
  
  
• Lymphadenitis (inflammation of the lymph nodes).
  
  
• Acute glomerulonephritis (kidney inflammation).
  
  
• Subacute bacterial endocarditis (heart inflammation).