Cellulitis is a bacterial skin infection that reaches into the deeper layers of skin and the connective tissue beneath them. It causes an area of skin to become red, swollen, warm, and painful, and it can spread if left untreated. Cellulitis is common: hospitalizations for it in the United States reached roughly 537,000 in 2013, nearly double the number from 1998, with annual hospital costs totaling $3.74 billion.
How Cellulitis Develops
Bacteria enter through a break in the skin, sometimes one so small you don’t notice it. A cut, scrape, surgical wound, insect bite, or cracked dry skin can all serve as entry points. Once inside, the bacteria move past the outer skin layer and into the deeper tissue, where they multiply and trigger inflammation. The infection can spread along tendons and muscles, and pus may form in the affected tissue.
The most common cause is group A streptococcus, the same family of bacteria behind strep throat. Staphylococcus aureus is the second most common culprit. In people with weakened immune systems or chronic wounds, less typical bacteria can be involved. Group A strep bacteria produce toxins that can drive a more aggressive, invasive infection in some cases.
What Cellulitis Looks and Feels Like
The hallmark signs are redness, swelling, warmth, and tenderness in a patch of skin that expands over hours or days. On lighter skin, the area often looks dark red or slightly purple, and the borders tend to be poorly defined, blending gradually into the surrounding skin rather than stopping sharply. On darker skin tones, redness can be harder to see, but the area will still feel noticeably warm, swollen, and painful to the touch.
Cellulitis most commonly affects the lower legs, though it can occur anywhere on the body. The skin may look tight or glossy as swelling increases. Some people develop blisters or dimpling of the skin surface. Mild cases may not cause a fever or general illness, but more serious infections can bring chills, fatigue, body aches, and swollen lymph nodes near the affected area. Red streaks extending outward from the infection suggest it is spreading through the lymph vessels.
Who Is Most at Risk
Anything that damages the skin barrier or weakens circulation raises the risk. Common factors include:
- Skin conditions: Eczema, athlete’s foot, and chronic dry or cracked skin create openings bacteria can exploit. Athlete’s foot between the toes is one of the most overlooked entry points for leg cellulitis.
- Swelling and poor circulation: Lymphedema (chronic tissue swelling) and venous insufficiency slow the body’s ability to fight infection locally. Fluid buildup in the legs stretches the skin and creates microscopic breaks.
- Obesity: Excess weight contributes to both swelling and skin fold moisture, increasing vulnerability.
- Weakened immune system: Diabetes, HIV, immunosuppressive medications, and advanced age all reduce the body’s defenses against bacterial invasion.
- Previous cellulitis: Having had cellulitis once significantly raises the chance of getting it again, often in the same area.
Cellulitis vs. Erysipelas
Erysipelas is a closely related skin infection that is frequently confused with cellulitis, but there are practical differences. Erysipelas only affects the outermost layers of skin and the lymph vessels. It produces a raised, shiny, clearly bordered area of redness. People with erysipelas typically develop a fever and feel sick right from the start.
Cellulitis, by contrast, reaches deeper. The redness is less sharply defined and often has a darker red or purple tone. Fever and general illness are less common with mild cellulitis, though they appear in more serious cases. Both infections require antibiotics, but recognizing the difference helps explain why cellulitis can sometimes be harder to detect early and why it carries a higher risk of spreading to deeper structures.
What Happens Without Treatment
Cellulitis does not resolve on its own. Without antibiotics, the infection can spread through the bloodstream, leading to sepsis, a potentially life-threatening body-wide inflammatory response. In rare cases, the bacteria can invade the tissue layers around muscles, a condition called necrotizing fasciitis that destroys tissue rapidly and requires emergency surgery. Bone infection is another possible complication when cellulitis sits near a joint or bone.
Warning signs that the infection is becoming dangerous include rapidly expanding redness, severe pain that seems out of proportion to what you see on the skin, skin that turns dusky or black, large blisters, a crackling sensation under the skin (caused by gas produced by bacteria), high fever, confusion, or a rapid heart rate. These symptoms call for emergency care.
How Cellulitis Is Treated
Oral antibiotics are the standard treatment for most cases. A typical course lasts 5 to 10 days, and you should start seeing improvement within 2 to 3 days. It is normal for redness to initially look slightly worse in the first 24 to 48 hours before it begins to fade, because the inflammation takes time to settle even as the bacteria are being killed. If the redness keeps expanding or your symptoms worsen after 2 to 3 days of treatment, your antibiotic may need to be changed.
More severe infections, especially those accompanied by fever, rapid spreading, or signs of sepsis, may require intravenous antibiotics in a hospital. Elevating the affected limb and keeping it rested helps reduce swelling and speeds recovery. Marking the edge of the redness with a pen can help you track whether the area is growing or shrinking over time.
Preventing Recurrence
Cellulitis has a frustrating tendency to come back, particularly in the legs. The infection itself damages lymph vessels, which worsens swelling, which in turn makes future infections more likely. Breaking this cycle matters.
Daily skin care is the foundation: keeping skin moisturized to prevent cracking, treating athlete’s foot promptly, cleaning cuts thoroughly, and managing chronic swelling with compression stockings when appropriate. For people who have had at least two episodes of leg cellulitis within three years, preventive antibiotics are an option. A Cochrane review of five trials found that prophylactic antibiotics reduced the risk of recurrence by 69% while people were taking them. The number needed to treat was six, meaning for every six people on preventive antibiotics, one additional person avoided a recurrence.
The protection comes with a significant caveat: once the antibiotics are stopped, the benefit fades. Recurrence rates return to near-baseline levels after discontinuation. The most common side effects of long-term preventive antibiotics are nausea, diarrhea, rash, and yeast infections. This makes the non-drug strategies, like controlling swelling and maintaining skin integrity, essential companions to any prevention plan.