Cellulitis in the legs is caused by bacteria entering through a break in the skin and infecting the deeper layers of tissue beneath the surface. The two most common culprits are group A streptococcus and staphylococcus aureus. The legs are the most frequently affected body part, and the infection strikes roughly 25 out of every 1,000 people per year.
How Bacteria Get In
Your skin is a barrier, and cellulitis starts when that barrier is compromised. The infection originates at sites where skin integrity has been disrupted: cuts, scrapes, surgical wounds, insect bites, animal bites, pressure ulcers, or injection sites. Sometimes no obvious entry point can be found at all.
For leg cellulitis specifically, one of the most overlooked entry points is between the toes. Athlete’s foot (a common fungal infection) creates tiny cracks and fissures in the skin that are invisible to the casual eye but wide open to bacteria. Clinicians evaluating leg cellulitis routinely check between the toes for exactly this reason. Dry, cracked heels, eczema patches, and even minor razor nicks on the shins can serve the same purpose.
Why the Legs Are Especially Vulnerable
Gravity works against the legs. Blood and fluid naturally pool in the lower extremities, and several common conditions make this worse. Venous insufficiency, where the veins in your legs struggle to push blood back up toward your heart, causes chronic swelling that stretches and weakens the skin. Peripheral arterial disease reduces blood flow to the legs, slowing the delivery of immune cells to fight off infection. Both conditions are widespread in middle-aged and older adults, which helps explain why cellulitis incidence is highest among people aged 45 to 64.
Lymphedema, or chronic swelling caused by a damaged or overloaded lymphatic system, is particularly dangerous. Your lymphatic system is responsible for draining excess fluid from tissues and clearing out bacteria and waste. When it’s not working properly, fluid accumulates, oxygen delivery to the skin drops, and the immune response in the area weakens significantly. This creates an environment where bacteria can establish an infection more easily and thrive once they do.
The Obesity and Diabetes Connection
People with obesity have roughly 2.7 times the odds of developing cellulitis compared to people at a healthy weight, based on a meta-analysis of seven studies. The reasons are both mechanical and immunological. Excess weight increases pressure on the veins and lymphatic vessels of the legs, promoting swelling and fluid retention. It also contributes to skin fold moisture, reduced circulation, and chronic low-grade inflammation that impairs immune defenses.
Diabetes compounds the problem. Elevated blood sugar damages small blood vessels over time, reducing circulation to the extremities. It also impairs the function of white blood cells, making it harder for your body to fight off bacterial invaders. People with diabetes are more likely to develop foot ulcers and neuropathy (loss of sensation), meaning a cut or blister on the foot or ankle can go unnoticed long enough for bacteria to take hold.
Other Risk Factors
Several additional conditions increase your chances of developing leg cellulitis:
- Previous cellulitis: A history of cellulitis is one of the strongest predictors of future episodes. Each infection damages the lymphatic system further, creating a cycle where recurrence becomes more likely over time.
- Varicose veins: These indicate underlying venous insufficiency and are associated with chronic swelling and skin changes in the lower legs.
- Immune suppression: Conditions or medications that weaken the immune system lower your ability to contain bacteria before they spread.
- Alcohol use and smoking: Both impair circulation and immune function, and heavy alcohol use is independently linked to cellulitis risk.
- Prior vein surgery: Removal of veins (saphenectomy) for grafting or other purposes can disrupt normal drainage in the legs.
What Happens Inside the Skin
Once bacteria penetrate the outer layer of skin, they reach the dermis and the fatty tissue beneath it. These deeper layers are warm, moist, and rich in nutrients, which allows bacteria to multiply rapidly. As they spread, the immune system responds with inflammation: the area turns red, swells, and becomes warm and painful to touch. This redness typically expands outward from the original site with smooth, indistinct borders.
The infection is overwhelmingly one-sided. If both legs are red and swollen, it’s more likely a different condition, such as stasis dermatitis, which is a chronic skin change caused by poor vein function. Stasis dermatitis tends to develop slowly over years, affects both legs, and often involves brown discoloration and flaky skin. Cellulitis, by contrast, comes on quickly (usually over hours to days), is almost always in one leg, and is often accompanied by fever or feeling generally unwell.
The Recurrence Cycle
Cellulitis has a troubling tendency to come back, particularly in the legs. Each episode of infection triggers inflammation that damages the lymphatic vessels in the affected area. This damage leads to tissue scarring and fibrosis, which reduces the lymphatic system’s ability to drain fluid and clear bacteria. The result is worsening swelling, which further compromises the skin barrier and immune defenses, setting the stage for the next infection.
This cycle can be difficult to break. Chronic swelling from repeated infections impairs cell nutrition and oxygen delivery to the tissue, making the skin increasingly fragile. Uncontrolled lymphedema also promotes fat deposition and progressive tissue changes that become harder to reverse over time. Managing the underlying swelling through compression, skin care, and treating conditions like athlete’s foot is one of the most effective ways to reduce recurrence.
Warning Signs of Serious Progression
Most cellulitis cases (about 74%) are treated in outpatient settings and resolve with oral antibiotics. But in rare cases, the infection can progress to life-threatening complications. Necrotizing fasciitis, a rapidly spreading infection that destroys deeper tissue, can develop from what initially looks like ordinary cellulitis.
Signs that should prompt immediate medical attention include pain that seems far worse than the skin appearance would suggest, areas of numbness or decreased sensation, fluid-filled blisters (especially ones that darken over time), a crackling feeling under the skin, and skin that turns dusky, purple, or black. These changes can progress within 24 to 48 hours as the infection cuts off blood supply to the affected tissue. Patients may also develop signs of sepsis: high fever, rapid heart rate, fast breathing, confusion, or a sudden drop in blood pressure.