If you ever had a skin condition in which an area of your skin was red, swollen, painful and warm, then you may have had cellulitis. This skin infection can occur anywhere in the body, but the initial condition is usually a break in the skin. Whether your skin has a cut, an abrasion, a puncture or a piercing, you could suffer from cellulitis. With the destruction of the skin's integrity, bacteria can enter and overwhelm the immune defenses, thereby causing an infection.

Causative Organisms

Cellulitis can be caused by the normal flora of the skin or by bacteria not naturally found on the skin surface. Normal flora of the skin includes the bacterial species Staphylococcus aureus and Streptococcus pyogenes. Exogenous bacteria, which are not part of the natural flora, include Haemophilus influenzae and Pseudomonas aeruginosa. A dog or cat bite may cause cellulitis secondary to the organism called Pasteurella multocida, whereas a saltwater fish bite may cause cellulitis secondary to the bacteria Erysipelothrix rhusiopathiae.

Note, however, that multiple types of bacteria may infect the skin at the same time. Such instances are complicated, and a doctor will usually prescribe stronger antibiotics to ensure eradication of the infection. When the skin is intact, these microorganisms will not be able to enter. However, once there is a break, these microorganisms are able to multiply and produce toxins, which in turn activate inflammatory mechanisms. With inflammation, you experience the typical symptoms of cellulitis, such as redness, swelling, soreness, warmth and pus from the skin surface. You could even suffer from fever, chills, and lymph node pain and enlargement.

Risk Factors

Preexisting skin conditions, such as eczema, skin ulcers, dermatitis and psoriasis, serve as risk factors because these destroy the integrity of the skin barrier. Fungal infections such as athlete's foot also increase the risk. When cellulitis occurs on top of a fungal infection, it is sometimes called a superinfection. Animal bites can cause inoculation of large amounts of exogenous bacteria into the skin, thereby increasing the chances of getting an infection.

When a person has spider veins or varicose veins, their risk of getting cellulitis is high. Spider veins and varicose veins indicate poor circulation and drainage. When bacteria enter through a break in the skin, they do not necessarily cause infection. If there is good circulation in that area, the white blood cells and immune factors will be able to neutralize the bacteria. However, if blood flow to that area is not good, the immune cells and factors would not reach the bacteria, resulting in cellulitis. Edema, or swelling of extremities, is also a risk factor that is related to poor circulation. Swelling of tissues impedes blood and lymph flow, leading to a higher risk for cellulitis. Poor blood and lymph flow also explains why obesity and pregnancy are risk factors for cellulitis.

Another risk factor is immunodeficiency, in which the immune state is much less functional than normal. The patient will not be producing enough immune substances and cells. Some diseases, such as diabetes mellitus, result in immunodeficient states. Old age, recent surgery, hospitalizations or transplants can attenuate the immune system, thereby predisposing a person to cellulitis.