Cellulitis is a serious infection that spreads under the skin, affecting soft tissues such as the skin itself and the fat underneath it. Bacteria are the most common culprits, but very occasionally a fungus is responsible.
Cellulitis is not contagious, but it can strike anyone with broken skin or damaged tissue. Cellulitis infections occur most commonly on the legs, arms, or face. Facial infections are found mostly in infants and seniors. In children, about 1 in 12 cases of facial cellulitis leads to meningitis.
Cellulitis occurs in different forms. Periorbital cellulitis occurs around the eye sockets. Erysipelas results in raised, firm, bright red patches of skin and is caused by Streptococcus bacteria. Necrotizing fasciitis is an infection that occurs below the skin.
Cellulitis usually starts with a break or tear in the skin. Many species of bacteria inhabit the skin's surface and can thrive underneath it if the barrier is broken to let them through. Bacteria can infect burns and bruises as well as cuts and scratches, puncture wounds, and dermatitis. Other traumas that can lead to infection include:
There are several bacteria that can cause cellulitis. Streptococcus and Staphylococcus are the two most common species. The actual type of bacteria causing the infection depends on the nature of the exposure, such as infections resulting from a cut, a cat or dog bite, or swimming in salt or fresh water with broken skin.
Cellulitis can sometimes start even on skin that hasn't been broken. This normally only occurs in people with weakened immune systems. People with diabetes, people with alcoholism, people taking corticosteroids, and others with compromised immune systems are at greatly increased risk of cellulitis and tend to get worse infections.
Necrotizing cellulitis (or necrotizing fasciitis), a rare but severe form of cellulitis, is the dreaded "flesh-eating disease" that occasionally grabs media headlines. There are two types. The first is due to a very aggressive strain of Group A streptococcus and can start anywhere on the skin. The second is due to a polymicrobial infection of different bacteria that often starts just forward of the anus following surgery or rectal abscesses. Necrotizing cellulitis starts as an extremely painful, red swelling that soon turns purple and then black as the skin and flesh die. However, the risk of getting necrotizing cellulitis is extremely small. The risk is a bit higher for people who are already sick or have a compromised immune system.
Uncomplicated cellulitis begins with a small area of skin that's red, glossy, painful, and warm to the touch, typically around a cut. It most often strikes the lower leg, but the arms, hands, feet, and groin are other likely sites.
Some types of bacteria can get into the lymphatic system, a network of tubes and nodes that carry disease-fighting cells and waste matter around the body. If this happens, the infection can travel under the skin and spread to other parts of the body.
Another type of cellulitis is erysipelas. The inflamed skin in erysipelas is not smooth but raised, often completely separated from the layer beneath by a pocket of gas created by the bacteria. There's often a fever a day or two before the rash.
The most severe complications of cellulitis are meningitis and blood poisoning (if the bacteria get into the bloodstream, called bacteremia). Meningitis can occur if the bacteria get inside the central nervous system (e.g., brain or spinal cord). The most likely route is through the eye socket. Cellulitis around an eyelid is called preseptal cellulitis. It causes a rash that might be painful or itchy. If the bacteria get behind the seal where the eye meets its socket, the condition is called orbital cellulitis. This is a medical emergency that causes symptoms of fever and impaired vision. It requires immediate treatment to prevent meningitis or eye damage.
If the infection gets into the lymphatic system or the bloodstream, red lines may be seen under the skin before the main rash appears. This may cause symptoms like fever, sweats, and nausea.