Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Cellulitis

What Is It?

Cellulitis is a serious bacterial infection of the skin. Bacteria break through the skin's protective outer layer, typically at the site of an injury, such as a cut, puncture, sore, burn or bite. Cellulitis can occur at the site of surgery, or where there is a catheter. Once beneath the skin surface, bacteria multiply and make chemicals that cause inflammation in the skin.

Cellulitis that is not caused by a wound or catheter most often occurs on the legs and feet. However, it can develop on any part of the body, including the trunk, arms and face. It often develops where there is edema (swelling), poor blood flow, or a skin rash that creates breaks in the skin, such as a fungus infection between the toes (athlete's foot).

Many types of bacteria can cause cellulitis. Most cases are caused by Streptococcus pyogenes (strep) or Staphylococcus aureus (staph). During the last several years, it has become more common for a strain of drug-resistant bacteria to cause cellulitis. This bacteria is named community-acquired methicillin-resistant staphylococcal aureus, or "community-acquired MRSA." Infections with this type of bacteria can lead to blistering of the skin or deep, more serious infections.

Less common bacteria varieties can cause infection after animal bites, puncture wounds through wet shoes, or wounds exposed to freshwater lakes, aquariums, or swimming pools. When cellulitis is located around an eye socket, it is named periorbital cellulites. Periorbital cellulitis is frequently caused by the bacteria Haemophilus influenza. Because infection around the eye can spread to the brain if it is not quickly treated with antibiotics, periorbital cellulitis requires prompt medical attention.

Medical conditions that are closely related to cellulitis include:

• Erysipelas, a skin infection that causes raised, firm, bright red patches of skin — Usually, it is caused by Streptococcus bacteria. Erysipelas occurs most often on an arm or leg that has been damaged by previous surgery or is chronically swollen due to poor lymph flow (lymphedema). Erysipelas also can develop on the face, typically across the bridge of the nose and upper cheeks.

• Necrotizing fasciitis, also known as "flesh-eating strep" — This is an infection of the tissues below the skin, rather than the skin itself. Often, the skin in the area is discolored and extremely painful. Fasciitis is a life-threatening infection that requires prompt medical attention.