Cellulitis is an infection of the skin and the tissues beneath it. It has nothing to do with the cosmetic problem cellulite.

It's usually caused by bacteria, most commonly of the streptococci or staphylococci groups, which normally live on the skin's surface. Damage to the skin (a spot, scratch or larger wound) allows bacteria to enter the deeper tissues and establish an infection.

However, any bacteria can cause cellulitis, and the initial damage to the skin may be so minor as to go unnoticed. Some people are more vulnerable to cellulitis, or more likely to get complications, especially those with diabetes, or those whose immune system isn't working well.

• Hot, red, swollen and painful area of the skin and deeper tissues
• Fever
• Shivers and chills, caused by release of toxins from the bacteria

There may be a well-defined line where the cellulitis stops, but this may spread as the infection progresses. Rarely, there's oozing of pus and fluid at the skin puncture site, but usually the infection is more diffuse in the tissues.

Sometimes a cellulitis infection can be trapped by certain 'compartments' in the tissues, for example, around a muscle or ligament, or in the 'pulp' at the tip of a finger. The pressure in that compartment increases, cutting off the blood supply (which may cause gangrene) and damaging nerves. This needs urgent treatment and decompression.

Another complication is blood poisoning or septicaemia, when bacteria gets into the blood stream. This can be rapidly fatal and needs very urgent treatment.

Fortunately, cellulitis usually responds well to antibiotics. Ideally, the skin and wound should be swabbed to test the kind of bacteria causing the problem, then a penicillin type of antibiotic is usually needed (or erythromycin for those who are allergic to penicillin). A long course of treatment (up to two weeks) may be necessary.

Cellulitis can be prevented by being careful about hygiene with wounds and breaks in the skin. If you cut or scratch yourself, or break the skin in some way, make sure the wound is cleaned and treated with an antiseptic, then covered with a clean dressing or plaster. If the skin around the wound becomes red and sore, get medical advice as soon as possible.