An abscess forms when your body walls off an infection, creating a pocket of pus made up of bacteria, dead tissue, and white blood cells. The most common cause is bacterial infection, particularly Staphylococcus aureus, though abscesses can also develop from parasites, foreign materials, or even certain medications. They can appear almost anywhere: under the skin, at the root of a tooth, or deep inside organs like the liver or lungs.
How an Abscess Forms Inside Your Body
The process starts when bacteria breach your skin or tissue through a cut, a clogged pore, or damaged tissue. Your immune system responds immediately, flooding the area with white blood cells called neutrophils. These cells rush from the bloodstream to the infection site, guided by inflammatory signals the surrounding tissue releases. The neutrophils attack and kill bacteria, but they also die in the process. That mixture of dead bacteria, dead neutrophils, and destroyed tissue is what becomes pus.
What turns a simple infection into an abscess is a kind of arms race between you and the bacteria. Staphylococcus aureus, the most common culprit, produces proteins that trigger your body to build a wall of fibrous tissue around the infection site. These bacterial proteins cause clotting factors to form a thick capsule at the edges of the infected area, essentially creating a barrier. This capsule traps the infection in place, but it also shields the bacteria from your immune cells and from antibiotics circulating in your blood. Over time, as more neutrophils arrive and more tissue breaks down, the pocket fills with pus and the abscess grows.
This walling-off mechanism is why antibiotics alone often can’t clear an abscess. The fibrous capsule that contains the infection also prevents drugs from reaching the bacteria inside. That’s the reason most abscesses need to be physically drained.
Skin Abscesses and Their Triggers
Skin abscesses are by far the most common type. In the United States, skin and soft tissue infections occur at a rate of roughly 78 per 1,000 people per year, generating about 9 million emergency room visits per decade. Community-associated MRSA, a strain of staph bacteria resistant to common antibiotics, is the single most frequent cause of these infections.
Bacteria usually enter the skin through small breaks you might not even notice: a nick from shaving, an insect bite, a splinter, or a hair follicle that gets clogged and inflamed. Boils are a familiar example. They start as a red, tender bump around a hair follicle and can enlarge into a full abscess over several days. When multiple boils cluster together and connect beneath the skin, the result is a larger, deeper abscess called a carbuncle.
Injecting drugs with non-sterile needles is another common route, pushing bacteria directly into deeper tissue. Surgical wounds can also become infected if bacteria are introduced during or after a procedure.
Dental Abscesses
Dental abscesses typically develop from tooth decay, trauma to a tooth, or a failed root canal. When the protective enamel breaks down, bacteria from the mouth enter the inner cavity of the tooth (the pulp) and set up an infection. Because the pulp is enclosed in hard walls of dentine, the growing infection has nowhere to expand. This compression is what causes the intense, throbbing pain characteristic of a tooth abscess.
The infection eventually tracks down through the root canal into the jawbone, where it can spread into surrounding soft tissue. Partially erupted wisdom teeth are another common source. Bacteria get trapped between the emerging tooth and the gum tissue covering it, creating an environment where infection can take hold easily.
Internal Organ Abscesses
Abscesses can form inside organs when bacteria travel through the bloodstream or spread from a nearby infection. Liver abscesses are a well-studied example. The most common pathway is bacteria leaking from the bowel into the blood vessels that feed the liver (the portal vein). Conditions like diverticulitis, gallbladder infections, and appendicitis can all send bacteria along this route. Infected bile ducts can also seed the liver directly.
In rarer cases, bacteria reach the liver from distant infections. A heart valve infection or kidney infection can release small clumps of bacteria into the bloodstream that lodge in the liver and form multiple small abscesses, which may merge into a single large one. Penetrating trauma, such as a stab wound, introduces bacteria directly, while blunt trauma can cause internal bleeding and tissue death that create conditions favorable for infection.
Brain abscesses follow similar principles. Bacteria can spread from a sinus or ear infection into adjacent brain tissue, or arrive through the bloodstream from an infection in the lungs or heart. Lung abscesses often develop after someone inhales bacteria into the airways, which is more likely during unconsciousness, heavy sedation, or severe intoxication when the normal cough reflex is suppressed.
Non-Bacterial Causes
Not every abscess involves bacteria. Parasites, particularly Entamoeba histolytica (the organism that causes amoebic dysentery), are a major cause of liver abscesses in tropical regions. Some parasitic infections also shift the immune system in ways that make the body more vulnerable to bacterial and fungal abscesses. People with acute schistosomiasis, for instance, develop higher rates of staph-related liver abscesses because the parasite suppresses the part of the immune response that normally fights bacteria.
Sterile abscesses, those with no detectable infection at all, can also occur. These form as an inflammatory reaction to foreign materials in the body: injected medications, surgical implants like biodegradable plates and screws, dermal fillers, or vaccine components. The body treats the substance as an invader and mounts the same walling-off response it would use against bacteria, producing a pocket of inflammation and fluid that looks and feels like an infected abscess but grows no organisms on culture.
Who Is Most at Risk
Diabetes is one of the strongest risk factors for recurrent skin abscesses. High blood sugar impairs the function of neutrophils, making it harder for your immune system to contain infections before they become walled-off pockets. People with HIV, those receiving chemotherapy, and anyone on medications that suppress the immune system face similar vulnerability.
Certain inherited immune disorders also predispose people to repeated abscesses from a young age. Chronic granulomatous disease, for example, leaves neutrophils unable to kill bacteria effectively, leading to deep abscesses in the skin, lungs, and lymph nodes. Underlying cancers, including lymphoma and leukemia, can also first reveal themselves through a pattern of recurrent skin abscesses.
Household spread matters too. Staph bacteria, including MRSA, pass easily between people who share towels, razors, or bedding. When one person in a household develops a staph abscess, close contacts often carry the same strain on their skin or in their nose, setting up a cycle of reinfection.
What Happens if an Abscess Goes Untreated
A small, superficial abscess sometimes drains on its own and heals without intervention. But when an abscess grows or sits in deeper tissue, the risks escalate. The infection can spread into surrounding tissue, causing cellulitis, a painful, expanding area of red, hot, swollen skin. MRSA infections are particularly aggressive at destroying tissue: the bacteria liquefy infected areas, increasing pressure until the overlying skin dies from lack of blood flow.
The most dangerous complication is when bacteria escape from the abscess into the bloodstream, a condition called septicemia. From there, the infection can seed new abscesses in distant organs, including the lungs, bones, and heart valves. About 21% of skin and soft tissue infections seen in emergency departments involve complications beyond the initial infection site. Necrotizing fasciitis, a rapidly spreading infection that destroys the connective tissue beneath the skin, is rare but can develop from an untreated abscess and progresses within hours.
Reducing the Risk of Recurrence
For people who get repeated staph abscesses, breaking the cycle requires more than treating each infection as it appears. The bacteria typically live on the skin and inside the nose between episodes, reinfecting the person (or their household contacts) when the opportunity arises.
A standard decolonization approach involves all household members applying an antibiotic ointment inside the nostrils twice daily for five days while also washing with an antimicrobial body wash daily during that same period. Bedding should be changed at the start and end of the regimen, and towels changed daily throughout. For people who continue to develop abscesses after this initial effort, a longer protocol of monthly nasal treatments combined with antimicrobial washes two to three times per week for three months may help.
Daily habits play a significant role as well. Avoiding shared towels, razors, and personal care items cuts down on bacterial transmission. Keeping fingernails short, skipping loofahs (which harbor bacteria), switching to pump or pour lotions instead of jar-based ones, and changing underwear and sleepwear daily all reduce the bacterial load on the skin. These measures sound simple, but in a randomized trial of 183 households dealing with recurrent staph infections, the combination of decolonization and hygiene changes significantly reduced reinfection rates.