Possible signs and symptoms of cellulitis, which usually occur on one side of the body, include:

• Red area of skin that tends to expand
• Swelling
• Tenderness
• Pain
• Warmth
• Fever
• Red spots
• Blisters
• Skin dimpling

Cellulitis occurs when bacteria, most commonly streptococcus and staphylococcus, enter through a crack or break in your skin. The incidence of a more serious staphylococcus infection called methicillin-resistant Staphylococcus aureus (MRSA) is increasing.

Although cellulitis can occur anywhere on your body, the most common location is the lower leg. Bacteria are most likely to enter disrupted areas of skin, such as where you've had recent surgery, cuts, puncture wounds, an ulcer, athlete's foot or dermatitis.

Animal bites can cause cellulitis. Bacteria can also enter through areas of dry, flaky skin or swollen skin.

Several factors put you at increased risk of cellulitis:

• Injury. Any cut, fracture, burn or scrape gives bacteria an entry point.
• Weakened immune system. Conditions that weaken your immune system — such as diabetes, leukemia and HIV/AIDS — leave you more susceptible to infections. Certain medications also can weaken your immune system.
• Skin conditions. Conditions such as eczema, athlete's foot and shingles can cause breaks in the skin, which give bacteria an entry point.
• Chronic swelling of your arms or legs (lymphedema). This condition sometimes follows surgery.
• History of cellulitis. Having had cellulitis before makes you prone to develop it again.
• Obesity. Being overweight or obese increases your risk of developing cellulitis.

Recurrent episodes of cellulitis may damage the lymphatic drainage system and cause chronic swelling of the affected limb.

Rarely, the infection can spread to the deep layer of tissue called the fascial lining. Necrotizing fasciitis is an example of a deep-layer infection. It's an extreme emergency.