Cellulitis is a common but potentially serious bacterial infection of the skin. Many types of bacteria live on our skin. So, when there’s a break or cut in it, whether it’s from an injury, bug bite or other irritation, bacteria has a chance to enter our bodies, potentially resulting in cellulitis.

The condition, which affects the middle and deep layers of the skin, causes pain, redness, and swelling; it’s most commonly found on the lower legs and feet. The infection usually appears as a small spot on the skin that quickly spreads to cover a much larger area.

About 14 million cases of cellulitis occur in the U.S. every year. It’s more common among adults ages 50 and older. Men and women are affected equally.

Antibiotics are prescribed to treat cellulitis, and most people recover without incident. More serious cases, however, may require hospitalization to manage the infection. About 650,000 people are hospitalized for the treatment of cellulitis each year. Seeking early treatment can prevent the infection from spreading, reducing the need for hospitalization during treatment.

Cellulitis is a bacterial infection of the skin that is likeliest to affect the lower legs and feet. It can cause pain and swelling, making it more difficult for people to complete their activities of daily living.

Bacteria lives on the surface of everyone’s skin. In a healthy person, the bacteria remains there without entering the body. But when a person has a break in the skin—either from an injury, surgery, insect bite, intravenous drug use, or something else—the bacteria has an opportunity to travel inside the body. And once inside, the bacteria can cause inflammation and infection.

People may be more susceptible to cellulitis if they have a chronic health condition, such as diabetes or vascular disease, or if they’re recovering from surgery. Individuals who are immunocompromised are also at increased risk of cellulitis.

Group A Streptococcus (a common type of strep) and Staphylococcus aureus (staph infection) are common causes of cellulitis. Other bacteria might be responsible for certain cases, but doctors typically do not identify the pathogen causing the infection unless the patient is not responding to treatment or it’s a complicated case (for instance, an abscess is present).

People who have cellulitis can experience such symptoms as:

• A rash or sore on the skin that appears suddenly and expands quickly
• Pain and swelling in the affected area
• Redness and warm skin in the affected area
• Muscle pain in the affected area
• Stiff joints in the affected area
• Skin that looks blistered or pitted or that appears to be taut and glossier than the surrounding skin
• Fever
• Chills
• Fatigue
• Nausea and vomiting
• Swollen lymph nodes

Cellulitis can worsen quickly over a short period of time. The affected skin area may grow larger within several hours or days.

People are at increased risk of developing cellulitis if they have:

• Cuts, scrapes, or other breaks in the skin
• Skin ulcers caused by diabetes or vascular disease
• Stitches or healing wounds from a recent surgery
• Insect bites or stings that have pierced the skin
• Animal (or human) bites that have broken the skin
• History of methicillin-resistant Staphylococcus aureus (MRSA)
• New piercings or tattoos
• Cracked or peeling skin on the feet or between the toes
• Athlete’s foot
• Eczema
• Diabetes
• Peripheral artery disease
• Shingles
• Chickenpox
• Lymphedema

Other risk factors include:

• Taking corticosteroids or other immune-suppressing medications
• Having an immune-suppressing condition, such as HIV/AIDS
• Having a history of peripheral vascular disease
• Injection drug usage

Doctors can diagnose cellulitis after hearing about a patient’s medical history and examining their skin. In some cases, diagnostic tests may be used. An early diagnosis can help people avoid a more complicated treatment and recovery.

During the medical history part of the exam, you should tell your doctor if you’ve had any cuts, injuries, or skin ulcers in the affected area—or if you’ve recently had surgery or a problem like athlete’s foot, or other infections.

During a physical exam, your doctor will examine the skin in the affected area, looking for telltale signs, including a red area with uneven edges, swelling, and skin that feels warmer to the touch than surrounding tissue.

The following diagnostic tests may also be used to diagnose cellulitis:

• Blood tests to look for the presence of bacteria and/or an elevated white blood cell count—a sign of infection
• Ultrasound imaging to see whether an abscess or buildup of pus is present and to check for blood clots.

However, the results of these tests may not be conclusive.

Antibiotics are used to treat cellulitis. Most patients can take oral antibiotics at home. A typical course of antibiotics targets group A strep and staph infections and lasts for a minimum of five days. For serious infections requiring a hospital stay, intravenous antibiotics might be necessary.

If cellulitis developed because of an existing condition, such as athlete’s foot, eczema, or diabetic foot ulcers, treating the underlying condition should help to improve a patient’s cellulitis.

With antibiotic treatment, most people recover from cellulitis at home. Others, however, will recover while in the hospital if they require intravenous antibiotics.

Some people experience complications of cellulitis, such as deep tissue infections or bacteria in their bloodstream. This is more likely among people who have had lymphedema. People with these conditions are also at increased risk of developing cellulitis infections in the future.

However, most people should expect a full recovery without complications.

This article was medically reviewed by Yale Medicine infectious diseases specialist Scott Roberts, MD.