A cyst is a closed sac filled with fluid or semi-solid material, while an abscess is a pocket of pus caused by an active infection. They can look similar from the outside, especially when they appear just under the skin, but they form for different reasons, feel different, and require different treatment. Knowing which one you’re dealing with changes what happens next.
What’s Inside Each One
A cyst is essentially a small, enclosed pocket lined by a wall of abnormal cells that are distinct from the tissue around them. Inside that sac, you’ll typically find fluid, air, or an oily substance called sebum that your skin naturally produces. Epidermal cysts, one of the most common types, often contain a thick, cheese-like material. Most cysts are not cancerous. They’re structural problems, not infections.
An abscess, by contrast, is your immune system’s battlefield. When bacteria (or less commonly, fungi or parasites) invade tissue, your body sends white blood cells to fight the infection. The resulting mixture of dead cells, bacteria, and tissue fluid is pus. That pus collects in a pocket, and the surrounding tissue becomes inflamed and swollen. Unlike a cyst, an abscess is always tied to an active immune response.
How They Form
Cysts develop slowly, often over weeks, months, or even years. They typically form when something blocks a duct or gland. A skin cyst might appear because a hair follicle gets plugged or a sebaceous gland’s opening is obstructed. In about 2 percent of women, the Bartholin’s glands near the vaginal opening can become blocked by irritation or minor injury, causing fluid to back up and create a cyst. Ovarian cysts form during the menstrual cycle when a follicle doesn’t release an egg or doesn’t shrink afterward. In all these cases, the problem is accumulation, not infection.
Abscesses form much faster, typically developing over days. A skin abscess might start after bacteria enter through a cut, scratch, or ingrown hair. A dental abscess forms when bacteria get into the soft inner tissue of a tooth through a crack, chip, or area of decay. Some abscesses develop internally near organs like the liver, kidneys, or even the brain, though internal abscesses are far less common than ones on or just below the skin.
How They Feel and Look
A cyst usually presents as a firm, round lump under the skin. It’s often painless unless it becomes very large or sits in a spot where it gets pressed or rubbed. The skin over a cyst typically looks normal in color and temperature. You can often feel the smooth, defined edges of the sac beneath the surface. Many people have cysts for years without any discomfort.
An abscess announces itself. The skin over it is red, warm, and swollen. It’s tender or outright painful, especially when touched. As pus accumulates, the center may feel soft or squishy compared to the firm edges around it. Abscesses can also cause symptoms beyond the local area: fever, chills, fatigue, and a general feeling of being unwell. If the abscess is in your mouth, you’ll likely notice throbbing pain that radiates into your jaw or ear.
One important wrinkle: a cyst can become infected and turn into an abscess. If a previously painless lump suddenly becomes red, hot, and painful, that’s a sign bacteria have gotten inside the cyst wall.
Where They Commonly Appear
Cysts show up most often on the face, neck, back, and behind the ears. Ovarian cysts are extremely common in women of reproductive age. Bartholin’s cysts appear near the vaginal opening. Ganglion cysts develop along tendons, usually on the wrist or hand.
Abscesses have a wider range. Skin abscesses frequently form on the neck, chest, face, buttocks, armpits, and around the anus or rectum (anorectal abscesses). Pilonidal abscesses develop in the crease of the buttocks. Breast abscesses can occur during breastfeeding. Dental abscesses form at the root of a tooth or in the gums. Less commonly, abscesses develop internally in the abdomen, spinal cord, or brain.
How Each One Is Treated
Treatment differs significantly because the underlying problem is different.
For abscesses, the standard treatment is incision and drainage. A healthcare provider makes a small cut, lets the pus drain out, and often packs the wound so it can continue draining and heal from the inside out. This is the primary treatment because antibiotics alone are not enough for most abscesses. The pus needs a way out. Antibiotics may be added alongside drainage, but they can’t replace it. For very small collections of pus, manual expression with antibiotics alone is sometimes sufficient.
For cysts, the approach depends on whether they’re causing problems. Many cysts need no treatment at all. If a cyst is bothersome, painful, or cosmetically concerning, the definitive treatment is complete surgical excision, meaning the entire sac wall is removed. This distinction matters: if a cyst is simply drained without removing the wall, it will very likely come back. A systematic review comparing surgical excision to simple drainage for epidermal cysts found that complete excision consistently had lower recurrence rates. Removing the wall eliminates the structure that produces and traps the fluid in the first place.
If a cyst has become infected (essentially turning into an abscess), the infection is usually treated first with drainage and possibly antibiotics, and then the cyst wall is removed in a separate procedure once the inflammation has settled.
How Doctors Tell Them Apart
A physical exam is often enough for skin lumps close to the surface. The color, warmth, pain level, and firmness of the lump give strong clues. When the diagnosis isn’t clear, or when the lump is deep inside the body, ultrasound is the go-to tool. On ultrasound, a simple cyst appears as a smooth, thin-walled sac with uniform fluid inside. An abscess shows thicker, irregular walls with cloudy internal material representing pus and cellular debris. Abscesses also tend to show increased blood flow in the surrounding tissue, a sign of active inflammation. Sometimes internal air pockets are visible within an abscess, another marker of infection.
Signs That Need Urgent Attention
Most cysts are harmless and can wait for a routine appointment. Abscesses, however, carry real risk if left untreated. The infection can spread into surrounding tissue or enter the bloodstream, potentially leading to sepsis. Warning signs that an infection is spreading include a fast heart rate, rapid breathing, high fever or unusually low body temperature, confusion, extreme fatigue, and warm or clammy skin. Low blood pressure and reduced urination are later signs that the body is under serious stress. A red streak extending outward from the abscess along the skin is another signal that the infection is moving beyond its original pocket.
An abscess that’s growing quickly, accompanied by fever, or located near sensitive structures like the eye, groin, or spine warrants prompt medical evaluation rather than a wait-and-see approach.