SSTI stands for skin and soft tissue infection, a broad category covering any bacterial infection that affects the skin, the fat beneath it, the connective tissue (fascia), or muscle. These infections range from minor and easily treatable, like a small boil, to life-threatening emergencies like necrotizing fasciitis. SSTIs are among the most common reasons people visit emergency departments and primary care offices.
Types of SSTIs
SSTIs fall into two main groups based on whether pus is involved. Purulent SSTIs produce a visible collection of pus. These include abscesses (pockets of infection under the skin), boils (infected hair follicles), and carbuncles (clusters of boils that merge together). Nonpurulent SSTIs don’t produce a drainable pocket of pus. Cellulitis and erysipelas are the most common examples. Cellulitis is a spreading infection of the deeper skin layers, while erysipelas affects the upper skin and has sharper, more defined borders.
This distinction matters because it changes how the infection is treated. Purulent infections typically need to be drained, while nonpurulent infections are treated primarily with antibiotics.
Within each group, infections are further classified as mild, moderate, or severe based on whether you’re showing signs of a bodywide response to infection: fever, rapid heart rate, rapid breathing, or abnormal white blood cell counts. Severe cases involve these systemic signs plus complications like dangerously low blood pressure or tissue death.
What Causes SSTIs
Two bacteria are responsible for the vast majority of skin and soft tissue infections: Staphylococcus aureus (staph) and streptococcal species (strep). Staph is the dominant cause of purulent infections like boils and abscesses, while strep is more commonly behind cellulitis and erysipelas.
MRSA, a drug-resistant form of staph, has become a significant concern. MRSA rates in skin infections vary widely by region, ranging from 5% to as high as 80% in some populations. More than half of staph-related skin infections are acquired in the community rather than in hospitals, meaning otherwise healthy people pick these up during everyday life. MRSA is especially common in people with a prior history of MRSA infection, those with chronic wounds, and people who have frequent contact with healthcare facilities.
More serious infections like necrotizing fasciitis can be caused by a single aggressive organism (often group A strep) or a mix of several bacteria working together. Burn patients and people with compromised immune systems may also develop infections from gram-negative bacteria that don’t typically cause skin problems in healthy people.
Symptoms to Recognize
The hallmark signs of an SSTI are redness, warmth, swelling, and pain at the affected area. These four findings together are the minimum criteria for diagnosing a skin infection. The skin may look shiny or feel tight. With purulent infections, you may notice a soft, fluid-filled area beneath the skin that feels like it could be squeezed, which indicates a pocket of pus forming.
Cellulitis typically appears as a spreading patch of red, warm skin that’s tender to touch. It can develop anywhere but commonly affects the lower legs. You might also notice red streaks extending from the area, which suggests the infection is spreading along lymphatic channels.
Warning Signs of a Dangerous Infection
Certain symptoms signal that a skin infection may be far more serious than it appears. Pain that seems dramatically worse than the visible skin changes suggest is one of the most important red flags for necrotizing fasciitis. This “pain out of proportion” to the exam is a classic early warning. Other signs of advanced disease include blisters or fluid-filled sacs on the skin, a crackling sensation when you press on the tissue (caused by gas produced by bacteria), areas that have gone numb instead of painful, dark or purplish discoloration, and visible skin death. Any of these warrants emergency medical evaluation.
Who Is Most at Risk
Several conditions make SSTIs more likely to develop and harder to treat. Diabetes is one of the strongest risk factors, affecting both susceptibility and outcomes. Other significant risk factors include obesity, peripheral vascular disease (poor circulation to the limbs), chronic kidney disease, liver cirrhosis, cardiovascular disease, immune suppression, and chronic open wounds. People who inject drugs are at particularly high risk due to repeated breaks in the skin under non-sterile conditions.
Obesity deserves special mention. Beyond raising the baseline risk for skin infections, it increases the likelihood of treatment failure in people hospitalized with cellulitis or abscesses, and it’s a risk factor for recurrent MRSA infections.
How SSTIs Are Treated
Treatment depends on whether the infection is purulent or nonpurulent and how severe it is.
For purulent infections like abscesses, the primary treatment is incision and drainage. A healthcare provider opens the abscess, allows the pus to drain, and often packs the wound to keep it open while it heals from the inside out. For mild cases, drainage alone may be enough. When there are signs of a systemic response (fever, rapid heart rate), antibiotics targeting staph are added. Severe cases with low blood pressure or failed initial treatment call for antibiotics specifically effective against MRSA.
For nonpurulent infections like cellulitis, antibiotics are the mainstay. Mild cellulitis without systemic symptoms is typically treated with oral antibiotics that target strep bacteria. The standard course is 5 days, extended if the infection hasn’t improved by then. Moderate cases with systemic signs need stronger antibiotic regimens, and severe cases, particularly those associated with penetrating injuries, IV drug use, or known MRSA exposure, require antibiotics that cover both MRSA and strep.
Superficial infections like impetigo can often be managed with a topical antibiotic ointment applied twice daily for 5 days. When lesions are widespread or part of an outbreak, oral antibiotics for 7 days are preferred to reduce the chance of spreading infection to others.
What Recovery Looks Like
One thing that catches many people off guard: cellulitis can actually look worse during the first 24 hours of antibiotic treatment. This happens because the rapid destruction of bacteria releases inflammatory substances that temporarily increase redness and swelling. This doesn’t mean the antibiotics aren’t working.
Meaningful improvement is generally expected within 48 hours of starting appropriate therapy. If there’s no improvement by that point, the diagnosis or antibiotic choice may need to be reconsidered. For infections that required IV antibiotics in the hospital, the typical benchmark for switching to oral medication and going home is being fever-free for 48 to 72 hours with visible clinical improvement.
Preventing Recurrence
Recurrent SSTIs are frustrating and common, especially when MRSA is involved. The most effective prevention strategy combines three elements: hygiene education, an antibiotic ointment applied inside the nostrils to eliminate staph colonization, and antiseptic body washes. In one study, this combination reduced recurrent infection rates to 11%, compared with 26% in people who received hygiene education alone.
Antiseptic body washes using chlorhexidine or dilute bleach baths are both effective options. Bleach baths typically involve adding a small amount of household bleach to a full bathtub and soaking for 5 to 15 minutes. General hygiene measures also help: keeping wounds covered, not sharing towels or razors, washing hands frequently, and laundering clothing and bedding in hot water during active infections.