Cysts form when fluid, air, or cellular debris gets trapped in a closed sac of tissue. The specific cause depends entirely on the type of cyst and where it develops, but most fall into a few broad categories: blocked ducts or follicles, hormonal fluctuations, physical injury, genetic mutations, infections, or underlying joint and organ conditions. Some cysts are completely harmless and resolve on their own. Others signal a deeper problem worth addressing.
Blocked Hair Follicles and Skin Injuries
The most common skin cysts, often called epidermal inclusion cysts, form when a hair follicle gets blocked at its opening. Skin cells that would normally shed and leave the body become trapped beneath the surface instead. These cells, along with a protein called keratin, collect in a pocket under the skin and gradually form a firm, round lump.
Skin injuries can trigger the same process. A scratch, surgical wound, or even chronic sun damage can disrupt the normal path skin cells take to reach the surface. The injury pushes surface cells down into the deeper layer of skin, creating a pocket where keratin builds up. This is why cysts sometimes appear at the site of an old scar or wound. On parts of the body without hair follicles, trauma is the primary driver. Acne is another common culprit, since inflamed or clogged pores create the same kind of blockage.
A related type, the pilar cyst, originates specifically from the outer root sheath of the hair follicle. These cluster heavily on the scalp because that’s where hair follicles are densest and most active.
Hormonal Changes and Ovarian Cysts
Most ovarian cysts are “functional” cysts, meaning they form as a normal part of the menstrual cycle. Every month, your ovaries grow small fluid-filled sacs called follicles. These follicles produce estrogen and progesterone and eventually rupture to release an egg during ovulation. When that process doesn’t go as planned, a cyst can develop.
Follicular cysts happen when the follicle fails to rupture at mid-cycle. Instead of releasing its egg, it keeps growing. Corpus luteum cysts form after an egg has been released. The opening where the egg came out seals shut, trapping fluid inside the now-empty follicle. Both types are usually painless and resolve within a few menstrual cycles without treatment.
Fertility medications that stimulate ovulation can increase the likelihood of these cysts forming, since they push the ovaries to produce more follicles than usual. Hormonal imbalances unrelated to medication can have the same effect, keeping follicles growing when they should have ruptured or shrunk back down.
Joint Stress and Fluid Buildup
Ganglion cysts are the rubbery lumps that commonly appear on wrists, hands, and feet. They grow out of a joint or the lining of a tendon and contain a thick, jelly-like fluid similar to the lubricating fluid found naturally in joints. No one has identified a definitive cause, though repetitive stress on a joint is a suspected trigger. They tend to show up more often in people who perform repeated wrist motions.
Baker’s cysts form behind the knee through a more clearly understood process. When the knee joint becomes inflamed, typically from osteoarthritis, rheumatoid arthritis, or a cartilage tear, it produces excess fluid. That fluid can push into a pouch at the back of the knee, creating a visible, sometimes painful swelling. Treating the underlying knee problem is usually the key to resolving the cyst itself.
Genetic Mutations
Some cysts are written into a person’s DNA. Polycystic kidney disease is the clearest example. Mutations in two specific genes, PKD1 and PKD2, cause thousands of fluid-filled cysts to develop in the kidneys over time. The disease is inherited in a dominant pattern, meaning you only need one copy of the mutated gene from one parent to develop it.
The severity depends partly on which gene is affected. Mutations in PKD1 tend to cause a more aggressive form of the disease with earlier kidney decline. Mutations in PKD2, particularly in women, typically produce a milder course with symptoms appearing later in adulthood. If a person inherits one mutated copy and then the second copy also mutates over time, cysts can grow faster and kidney function can decline more steeply.
At the cellular level, these mutations disrupt calcium signaling inside kidney cells. When that signaling breaks down, cells that line the kidney’s tiny tubes begin to multiply abnormally. Rather than forming a solid mass, they create an outpouching that eventually pinches off from the tube and fills with fluid. The cyst then expands through two forces: the cells lining it keep dividing, and they actively pump fluid into its center.
Infections and Parasites
Certain infections produce cysts as part of the organism’s life cycle inside the human body. Cystic echinococcosis, also called hydatid disease, is caused by accidentally swallowing eggs from the tapeworm Echinococcus granulosus. Dogs that eat home-slaughtered livestock carry this tapeworm, and its eggs can be found in their stool. Once swallowed, the eggs hatch into larvae that travel through the bloodstream and form large, slow-growing cysts, most often in the liver and lungs. These cysts can take years to produce symptoms and can grow to several centimeters in diameter.
The tapeworm eggs are remarkably hardy, surviving snow and freezing temperatures in the environment. This makes echinococcosis more common in rural regions where people live in close contact with livestock and dogs, though cases occur worldwide.
How Cysts Differ From Abscesses
People sometimes confuse cysts with abscesses, but the two are fundamentally different. A cyst is a closed sac that contains fluid, air, or semi-solid material like the oily substance that naturally protects your skin. It forms gradually and is not caused by infection in most cases.
An abscess is a pocket of pus created by a bacterial infection. White blood cells rush to the infected area, fight the bacteria, and die in the process. The resulting mixture of living and dead cells, bacteria, and damaged tissue accumulates as pus. Abscesses are typically red, warm, and painful. Cysts can become infected and turn into something resembling an abscess, but an uninfected cyst is usually painless or only mildly tender and grows slowly over weeks or months rather than days.
Why the Same Body Can Develop Different Cysts
Because cysts arise from so many different mechanisms, it’s entirely possible to develop unrelated cysts in different parts of your body at different times. A hormonal ovarian cyst at 25, a ganglion cyst on your wrist at 35, and a skin cyst at 50 would each have a completely separate cause. The word “cyst” describes a structure, not a single disease. What they share is the basic architecture: a sac with a defined wall, filled with something that isn’t supposed to be there, growing in a space where it doesn’t belong.
The cause determines the outlook. Functional ovarian cysts almost always disappear on their own. Epidermal cysts can sit unchanged for years or be removed with a minor procedure if they bother you. Genetically driven kidney cysts require long-term monitoring because they progressively affect organ function. Knowing what type of cyst you have, and what triggered it, is the most important step in understanding whether it needs attention or can simply be left alone.