The main cause of cellulitis is bacteria entering the skin through a break or crack in its surface. Group A Streptococcus is responsible for roughly 65% of cases, making it the single most common culprit. Staphylococcus aureus accounts for about 14%, with gram-negative bacteria causing around 11% of the remainder.
How Bacteria Get Through the Skin
Healthy, intact skin is a remarkably effective barrier. Cellulitis develops when that barrier is compromised and bacteria that normally live harmlessly on the skin’s surface find a way in. The entry point doesn’t have to be dramatic. A small cut, a scrape, a puncture wound, a surgical incision, or even a burn can be enough. Skin conditions like eczema, athlete’s foot, and shingles create tiny cracks that serve as open doors for bacteria, sometimes without you ever noticing the break.
Dry, flaky, or swollen skin is also vulnerable. This is why people with chronic swelling in the legs are at particularly high risk. The skin stretches and weakens, and microscopic breaks form that bacteria can exploit.
What Happens Once Bacteria Get In
The infection follows a predictable three-step process. First, bacteria attach to cells in the deeper layers of the skin. Then they begin multiplying, invading tissue while evading the immune system. Finally, they release toxins that cause the hallmark symptoms: redness, warmth, swelling, and pain that spreads outward from the original site. The infection targets the dermis and the fatty tissue just beneath it, which is why cellulitis typically looks and feels like a deep, expanding area of inflammation rather than a surface wound.
Unlike an abscess, which walls off infection into a pocket of pus, cellulitis spreads diffusely through tissue. This spreading pattern is what makes it potentially serious if left untreated.
The Role of MRSA
While Group A Streptococcus dominates overall cellulitis cases, methicillin-resistant Staphylococcus aureus (MRSA) has become a significant factor in skin infections broadly. CDC data showed that among all skin and soft tissue infections in North America, Staph aureus was the most common cause at 44.6%, and about 36% of those isolates were methicillin-resistant. In some urban emergency departments, MRSA accounted for up to 64% of skin and soft tissue infections.
MRSA is more commonly associated with abscesses than with the spreading, non-purulent form of classic cellulitis. But when it does cause cellulitis, it matters because it resists standard antibiotics, requiring different treatment approaches.
Risk Factors That Make Cellulitis More Likely
Certain conditions dramatically increase your chances of developing cellulitis, even from minor skin breaks.
- Lymphedema: Chronic swelling from damaged lymph drainage is likely the single biggest risk factor. In a study of over 1,800 lymphedema patients, 37.6% had experienced at least one episode of cellulitis, and nearly a quarter had recurrent infections.
- Obesity: Higher body mass index shows up as an independent risk factor across multiple studies, likely because excess weight impairs circulation and puts more pressure on the skin of the lower legs.
- Venous insufficiency: Poor blood flow back to the heart causes fluid to pool in the legs, creating the kind of swollen, fragile skin that bacteria penetrate easily.
- Fungal skin infections: Athlete’s foot and similar fungal conditions create cracks between the toes that serve as common bacterial entry points, particularly for lower-leg cellulitis.
- Previous cellulitis: Having one episode significantly raises the odds of another. Between 35% and 47% of patients hospitalized for cellulitis have had a prior episode.
Why Cellulitis Keeps Coming Back
Recurrence rates are high. Between 16% and 53% of people who get cellulitis will develop it again within three years. The reasons are straightforward: the underlying risk factors tend to be chronic conditions that don’t resolve on their own.
Lymphatic dysfunction appears to be the primary driver of recurrence. Each episode of cellulitis can further damage the lymphatic system, creating a cycle where infection causes more swelling, which causes more skin fragility, which invites more infection. Chronic skin conditions like eczema, venous disease, and fungal infections between the toes also persist, keeping the door open for bacteria to re-enter.
Less Common Causes
In people with weakened immune systems, cellulitis can be caused by organisms that rarely affect healthy individuals. These include gram-negative bacteria like Pseudomonas and Serratia, certain fungi like Cryptococcus, and in rare cases even herpes simplex virus. These causes are uncommon enough that they’re primarily a concern for people on immunosuppressive medications or undergoing chemotherapy.
Water exposure creates its own category of risk. Cuts or scrapes that come into contact with warm saltwater can become infected with Vibrio vulnificus, a bacterium that causes rapidly progressing cellulitis and can become life-threatening quickly. Freshwater exposure carries risk from Aeromonas and other aquatic organisms. Fishermen and others who work in or around water with open skin wounds are particularly vulnerable.
Conditions Commonly Mistaken for Cellulitis
One striking finding from recent research is how often cellulitis is misdiagnosed. A systematic review found that across clinical studies, about 41% of patients diagnosed with cellulitis actually had something else. The range was wide, from 19% to 83% depending on the setting, but the pattern was consistent: cellulitis is frequently overcalled.
More than half of the misdiagnoses turned out to be three conditions: stasis dermatitis (skin inflammation from poor circulation), eczematous dermatitis, and simple edema or lymphedema. All three cause redness and swelling in the lower legs that can look remarkably similar to cellulitis. The key differences are that these conditions are typically present in both legs, develop gradually, and don’t cause fever, while cellulitis is almost always one-sided, progresses over hours to days, and often comes with systemic signs like warmth and feeling unwell.