Cellulitis is a bacterial skin infection that reaches into the deeper layers of skin and the tissue just beneath it. It causes an area of skin to become red, swollen, warm, and painful, and it can spread quickly if untreated. The infection almost always affects just one side of the body, most commonly a leg, and it often starts at a spot where the skin has been broken, even by something as minor as a small cut or a crack between the toes.
How Cellulitis Develops
Your skin is your body’s primary barrier against bacteria. When that barrier is broken, bacteria that normally live harmlessly on the skin’s surface can slip through and reach the deeper tissue underneath. The break doesn’t have to be dramatic. A paper cut, an insect bite, a surgical incision, a blister, or even dry, cracked skin can provide enough of an opening. One of the most overlooked entry points is athlete’s foot, a common fungal infection that causes tiny fissures between the toes. Those small cracks are a frequent starting point for cellulitis in the lower legs.
Once bacteria get past the skin barrier, they trigger an aggressive inflammatory response. The area swells as the immune system rushes blood and fluid to fight the invaders. This is what produces the hallmark redness, heat, and tenderness. Without treatment, the infection can continue spreading outward through the tissue.
The Bacteria Behind It
Two families of bacteria cause the vast majority of cellulitis cases. Group A Streptococcus is one of the most common culprits, according to the CDC. Staphylococcus aureus, including drug-resistant strains like MRSA, is the other major player. These bacteria are widespread in the environment and on human skin, which is why even everyday skin injuries can lead to infection under the right circumstances.
In most cases, doctors treat cellulitis based on its appearance without needing to identify the exact bacterial species involved. Standard antibiotic treatment typically lasts about 5 days, though courses may be extended if the infection hasn’t improved by then.
What Cellulitis Looks and Feels Like
The affected area is swollen, warm, painful to touch, and visibly red or discolored. The redness typically has smooth, indistinct borders that spread outward rather than forming a sharp line. Some people develop spots on the skin, blisters, or a dimpled texture sometimes compared to orange peel. Beyond the skin itself, cellulitis frequently causes systemic symptoms: fever, chills, and a general feeling of being unwell. If you notice the redness expanding noticeably over hours, that’s a sign the infection is progressing and needs prompt attention.
One important clinical feature: cellulitis is almost always one-sided. A symmetric or bilateral pattern, where both legs look the same, typically points to something else entirely.
Conditions That Raise Your Risk
Anything that compromises skin integrity or weakens circulation in the legs increases susceptibility. The most common risk factors include:
- Lymphedema or chronic swelling: Fluid buildup stretches the skin and impairs local immune defenses.
- Venous insufficiency: Poor blood return from the legs causes chronic swelling and skin breakdown.
- Obesity: Excess weight contributes to both lymphedema and skin fold moisture, creating environments where bacteria thrive.
- Diabetes or immunosuppression: A weakened immune response makes it harder to contain bacteria once they enter the skin.
- Previous cellulitis: A history of cellulitis is one of the strongest predictors of future episodes. After a first infection, about 6% of people experience a recurrence within 12 months. After a second episode, the 12-month recurrence rate jumps to 17%. After a third, it reaches nearly 30%.
- Prior surgery involving lymph nodes or veins: Procedures like mastectomy with lymph node removal or vein stripping for varicose veins can permanently alter drainage in a limb.
Conditions That Look Like Cellulitis
Cellulitis is frequently misdiagnosed because several other conditions cause red, swollen legs. Understanding the differences matters, because the wrong diagnosis means unnecessary antibiotics or a missed underlying problem.
Stasis dermatitis is the most common mimic. It results from chronic poor circulation in the legs and looks strikingly similar to cellulitis, with redness, swelling, and sometimes weeping skin. The key differences: stasis dermatitis is usually bilateral (affecting both legs), develops gradually over months or years, and the legs are generally not tender. A long, slow course and failed antibiotic treatments are strong clues that the problem is venous stasis rather than infection.
Contact dermatitis, both irritant and allergic types, can also cause alarming redness and swelling. The giveaway is the pattern: contact dermatitis tends to have sharp, well-defined borders matching the area that touched the irritant or allergen, and it often itches more than it hurts. Recent changes in soap, laundry detergent, or topical products usually point to the cause.
Lymphedema produces unilateral swelling that can look like cellulitis, but the edema is typically non-pitting (pressing a finger into the skin doesn’t leave a lasting dent), develops without fever, and causes thickened, warty-textured skin changes over time.
When Cellulitis Becomes Dangerous
Most cellulitis resolves well with oral antibiotics. Rarely, however, the infection can progress to more serious conditions. The most feared complication is necrotizing fasciitis, a rapidly destructive infection of the deeper tissue layers. Warning signs that suggest this progression include intense pain that seems out of proportion to what the skin looks like, areas of numbness, large fluid-filled blisters, a crackling sensation under the skin, and dark or dusky discoloration. Patients can quickly develop sepsis, shock, and organ failure. Amputation is a common outcome when necrotizing fasciitis affects a limb.
These complications are uncommon, but the speed at which necrotizing fasciitis progresses makes early recognition critical. Systemic symptoms that seem disproportionate to the visible skin changes, especially rapid deterioration, should prompt emergency evaluation.
Preventing Recurrence
Because cellulitis recurs at high rates, prevention is a practical priority for anyone who has had it. The most effective strategies target the underlying conditions that allowed the first infection.
Managing leg swelling is the single most impactful step. Compression stockings, leg elevation, and treating fluid retention all reduce the tissue environment that bacteria exploit. One clinical trial found that consistent use of compression therapy reduced the risk of recurrent leg cellulitis by roughly 80% over a follow-up period of up to three years.
Treating athlete’s foot matters more than most people realize. Applying antifungal cream to fungal infections between the toes eliminates one of the most common bacterial entry points. Keeping skin moisturized with emollients prevents the dry, cracked skin that also serves as a gateway. For people with repeated episodes despite these measures, long-term low-dose preventive antibiotics are sometimes used to break the cycle.