Cellulitis is a spreading infection that usually begins as a small area of tenderness, swelling, and redness on the skin. As this red area begins to spread, the person may develop a fever, sometimes with chills and sweats, and swollen lymph nodes (or glands) near the area of infected skin.
Unlike impetigo, which is a very superficial skin infection, cellulitis refers to an infection involving the skin's deeper layers; the dermis and subcutaneous tissue. The main bacteria involved in cellulitis is Staphylococcus ("staph"), the same bacteria that causes many cases of impetigo. Occasionally, other bacteria may cause cellulitis as well.
Some cases of cellulitis appear on areas of trauma, where the skin has broken open, such as the skin near ulcers or surgical wounds. Many times, however, cellulitis occurs where there has been no break in the skin at all. In such cases, it is anyone's guess where the bacteria came from. Patients who have diabetes or impairment of the immune system (for example, from HIV/AIDS or from drugs that suppress the immune system) are particularly prone to developing cellulitis.
The signs of cellulitis are those of any inflammation; redness, warmth, swelling, and pain. Any skin wound or ulcer that exhibits these signs may be developing cellulitis.
Other forms of noninfected inflammation may mimic cellulitis. People with poor leg circulation, for instance, often develop scaly redness on the shins and ankles; this is called "stasis dermatitis" and is often mistaken for the bacterial infection of cellulitis.
Staph (Staphylococcus aureus) is the most common bacteria that causes cellulitis.
Strep (Group A Streptococcus) is the next most common bacteria that causes cellulitis. A form of rather superficial cellulitis caused by strep bacteria is called erysipelas; it is characterized by spreading hot, bright red circumscribed area on the skin with a sharp raised border. The so-called "flesh-eating bacteria" are, in fact, also a strain of strep which can -- in severe cases -- destroy tissue almost as fast as surgeons can cut it out.
Cellulitis can be caused by many other types of bacteria. In children under age 6, H. flu (Hemophilus influenzae) bacteria can cause cellulitis, especially on the face, arms, and upper torso.
Cellulitis from a dog or cat bite or scratch may be caused by the Pasturella multocida bacteria, which has a very short incubation period of only four to 24 hours.
Cellulitis after an injury from a saltwater fish or shellfish (like a fish bite, a puncture from a fish spine, or a crab pinch) can be due to the Erysipelothrix rhusiopathiae bacteria. These same bacteria can also cause cellulitis after a skin injury on a farm, especially if it happened while working with pigs or poultry.