An abscess is a pocket of pus that forms when your body tries to wall off an infection. It can develop almost anywhere: under the skin, around a tooth, near the anus, or deep inside organs like the liver or brain. The pus inside is a mixture of dead white blood cells, destroyed tissue, and bacteria, all enclosed in a firm barrier your immune system builds to keep the infection from spreading further.
How an Abscess Forms
When bacteria enter your body through a cut, a clogged pore, or damaged tissue, your immune system sends white blood cells called neutrophils to the site. Neutrophils make up 60 to 70 percent of your white blood cells, and they’re your front line against bacterial invaders. They attack bacteria using toxic chemicals and an acidic environment that kills many microbes on contact.
But this fight produces collateral damage. As neutrophils destroy bacteria, they also destroy some of the surrounding tissue. Dead cells, spent white blood cells, and bacterial debris accumulate into a thick, yellowish fluid: pus. Your body then forms a wall of tissue around this material, creating a sealed-off cavity. That cavity is the abscess. The wall serves a protective purpose, preventing the infection from leaking into surrounding tissue or the bloodstream, but it also makes it harder for your immune system (and antibiotics) to fully clear the infection from the inside.
What Causes Them
The most common culprit is Staphylococcus aureus, a bacterium that lives on the skin of many healthy people. A large study across 11 U.S. emergency departments found that 59 percent of staph-related skin infections were caused by MRSA, the antibiotic-resistant strain. Skin abscesses are, in fact, the most common type of infection caused by community-acquired MRSA.
Other bacteria can cause abscesses too, especially in deeper tissues where infections tend to involve multiple bacterial species working together. Anything that lets bacteria past your skin’s natural barrier can set the stage: a scratch, an insect bite, an ingrown hair, a surgical incision, or even a blocked sweat gland. People with weakened immune systems, diabetes, or chronic skin conditions are at higher risk.
Where Abscesses Develop
Skin abscesses are the most common and the most visible. They frequently show up in the armpits, groin, buttock crease (pilonidal abscesses), and around the anus (perianal abscesses). Breast abscesses can develop during breastfeeding when bacteria enter through cracked skin on the nipple.
Dental abscesses form around the root of a tooth or in the gums, usually from untreated cavities or gum disease. Abscesses can also form in the throat, including between the tonsils and the throat wall (known as a quinsy or peritonsillar abscess).
Internal abscesses are far less common but more serious. They can develop in or near the liver, kidneys, pancreas, spinal cord, or brain, often as a complication of surgery, a spreading infection, or a condition like appendicitis or diverticulitis.
Signs and Symptoms
A skin abscess typically announces itself with five hallmark signs: pain, heat, swelling, tenderness, and redness. As it grows, the center may become soft and feel like it’s filled with fluid. This soft, squishy quality is what doctors call “fluctuance,” and it’s a key sign that pus has collected beneath the surface. In some cases, the skin over the center thins out and turns white or yellow as the abscess gets closer to rupturing on its own.
Fever can accompany a skin abscess, especially if the surrounding skin is also infected (cellulitis). Deep abscesses inside the body cause different symptoms: localized pain and tenderness, along with fever, fatigue, loss of appetite, and sometimes unexplained weight loss. Because you can’t see or feel a deep abscess directly, these systemic symptoms may be the only clue something is wrong.
How Abscesses Are Diagnosed
Most skin abscesses are diagnosed by physical exam alone. A doctor can usually tell by looking at and pressing on the area. When it’s not clear whether a swollen, red patch of skin contains a drainable pocket of pus or is simply infected tissue (cellulitis), ultrasound resolves the question quickly. On ultrasound, an abscess shows up as a distinct dark pocket of fluid surrounded by swollen tissue, while cellulitis appears as a “cobblestoned” pattern of swollen fat without a defined fluid collection.
For suspected internal abscesses, imaging is essential. CT scans are accurate for most locations, though MRI is generally more sensitive and may be preferred for abscesses near the brain or spinal cord.
Treatment: Drainage Is the Priority
The core treatment for nearly every abscess is drainage. Antibiotics alone can’t reliably penetrate the walled-off cavity, so the pus needs a way out. For skin abscesses, this means a procedure called incision and drainage. A doctor numbs the area, makes a cut over the center of the abscess, and lets the pus drain. They then use a small instrument to break up any internal pockets so all the infected material can escape, followed by flushing the cavity with sterile saline.
For abscesses larger than about 5 centimeters (roughly 2 inches), the cavity may be loosely packed with gauze to keep the wound open and allow continued drainage. Smaller abscesses generally don’t need packing, which is good news since packing can be uncomfortable without improving outcomes.
Antibiotics are not always necessary. For a straightforward skin abscess in an otherwise healthy person, drainage alone is often enough. Antibiotics are added when there are signs of infection spreading beyond the abscess, such as fever above 38°C (100.4°F), rapid heart rate, or significant surrounding redness. They’re also used for people with weakened immune systems or when an abscess keeps coming back. Recurrent abscesses at the same site should be cultured to identify the specific bacteria, and a doctor may look for an underlying cause like a cyst or foreign material.
Aftercare at Home
After drainage, keeping the area clean is the most important thing you can do. Change your bandage at least once a day, or whenever it gets wet or dirty. Applying warm, dry compresses or a heating pad on low three or four times a day helps with pain and encourages continued drainage. Always place a cloth between the heat source and your skin.
If your wound was packed with gauze, you’ll need follow-up appointments to have it changed or removed. Once the packing is out, soaking the area in warm water for 15 to 20 minutes twice a day helps the wound close from the inside out. This “inside out” healing is intentional: if the skin surface closes first, pus can reaccumulate underneath and the abscess can return.
What Happens if an Abscess Goes Untreated
A small abscess may occasionally drain on its own and heal without intervention. But many do not, and waiting carries real risks. The infection can spread into the surrounding soft tissue, causing cellulitis. In more serious cases, bacteria can enter the bloodstream and trigger sepsis, a life-threatening condition marked by a sudden drop in blood pressure, rapid heart rate, and widespread inflammation.
Perianal abscesses carry a particularly dangerous risk. If not adequately drained, they can progress to a severe form of tissue death where blood vessels clot off, cutting oxygen supply to surrounding skin and muscle. This causes rapid, aggressive tissue destruction that requires emergency surgery and carries significant risk of death. The warning signs of this kind of deterioration include sudden worsening pain that spreads beyond the original site, skin that turns dark or develops blisters, and any combination of fever with a rapid pulse and dropping blood pressure.
Even in less extreme scenarios, an undrained abscess can form a fistula, an abnormal tunnel between the abscess cavity and the skin surface or another organ. Fistulas often require surgical repair and tend to recur.