Cysts form when fluid, air, or semi-solid material gets trapped inside a pocket of tissue. The specific cause depends on the type of cyst, but the most common triggers fall into a handful of categories: blocked ducts or glands, hormonal fluctuations, injury, infection, and genetics. Most cysts are harmless, and many resolve on their own.
Blocked Glands and Ducts
The single most common reason a cyst develops is a blockage. Your skin is covered in tiny glands that produce oil (sebum) to keep the surface lubricated. If one of those glands or its drainage duct gets clogged, sebum has nowhere to go. It pools beneath the surface, and the body walls it off in a sac, forming a cyst. This blockage can happen because of abnormal cell shedding, a minor skin injury, or simply genetics. It has nothing to do with hygiene.
A similar process happens with epidermoid cysts, the firm, round lumps that commonly appear on the face, neck, or trunk. Your outer layer of skin constantly sheds dead cells. Sometimes those cells move deeper into the skin instead of flaking off, often because of irritation or damage to a hair follicle. Once trapped, the cells form the walls of a small pocket and begin secreting keratin, a thick protein. That keratin fills the interior and gives the cyst its characteristic dense, cheese-like contents.
Duct blockage isn’t limited to skin. In the breast, milk ducts can widen with age, and their walls may thicken and fill with fluid. The duct becomes clogged with a sticky substance, leading to swelling and sometimes infection.
Hormonal Changes
Ovarian cysts are overwhelmingly driven by normal hormonal cycling. Each month, the ovaries grow small fluid-filled sacs called follicles that produce estrogen and progesterone. A follicle is supposed to rupture and release an egg during ovulation. When it doesn’t, it keeps growing and becomes a follicular cyst.
A second type, called a corpus luteum cyst, forms after the egg has been released. The empty follicle normally shrinks and produces hormones to support a potential pregnancy. But if the opening seals shut, fluid accumulates inside, and the structure balloons into a cyst. Both of these are called functional cysts, and they’re extremely common. Most disappear within a few menstrual cycles without treatment.
Hormonal medications can raise the risk. Fertility drugs that stimulate ovulation push the ovaries to produce more follicles than usual, increasing the chances that one or more will fail to rupture properly. Conditions like polycystic ovary syndrome (PCOS), where hormonal imbalances interfere with regular ovulation, also lead to clusters of small cysts on the ovaries.
Skin Injuries and Friction
Trauma to the skin is a well-established trigger for cyst formation. When the skin is cut, scraped, or chronically irritated, it can disrupt the structure of a hair follicle or oil gland. The damaged tissue heals in a way that traps surface cells beneath the skin, where they continue to grow and produce keratin in an enclosed space. This is why cysts sometimes appear along surgical scars or in areas prone to repeated friction.
Cysts on the palms and soles, where hair follicles don’t exist, are thought to arise from direct implantation of skin cells into deeper tissue during an injury. Some cases have also been linked to HPV infection at the site. A small number of epidermoid cysts are congenital, forming before birth when skin cells get trapped along the lines where tissues fuse during embryonic development.
Joint Stress and Repetitive Motion
Ganglion cysts are the rubbery lumps that pop up on wrists, hands, and feet, typically near a joint or tendon. They’re filled with a thick, jelly-like fluid similar to the lubricating fluid found inside joints. The exact mechanism is still unclear, but joints or tendons that have been injured are more likely to develop them. People who perform repetitive hand or wrist movements often notice them, and they can fluctuate in size over time, sometimes disappearing on their own.
Parasitic Infections
Certain infections produce cysts inside the body as part of a parasite’s life cycle. The most significant example is echinococcosis, or hydatid disease, caused by larval stages of tapeworms in the genus Echinococcus. Humans typically become infected by accidentally ingesting tapeworm eggs from contaminated food, water, or contact with infected animals. Once inside the body, the larvae migrate to organs like the liver or lungs and develop into thick-walled cysts that enlarge gradually over months or years. These cysts produce daughter cysts internally and can grow large enough to cause pressure symptoms. If a cyst ruptures, the released material can seed secondary cysts throughout the body.
This type of cyst is rare in developed countries but remains a significant health concern in parts of South America, the Mediterranean, Central Asia, and East Africa, particularly in communities with close contact between livestock and dogs.
Genetic Conditions
Some people are genetically predisposed to forming cysts across multiple organs. Von Hippel-Lindau syndrome is one of the clearest examples. It’s caused by a mutation in a tumor suppressor gene that normally keeps cells from growing and dividing too rapidly. When this gene is altered, cells can proliferate in an uncontrolled way, forming both tumors and fluid-filled cysts in the kidneys, pancreas, and genital tract. The condition is inherited in an autosomal dominant pattern, meaning a single copy of the mutated gene is enough to raise the risk. Cyst and tumor formation then occurs when the second, healthy copy of the gene is lost in a specific cell during a person’s lifetime.
Polycystic kidney disease is another inherited condition where clusters of cysts grow within the kidneys over time, gradually replacing healthy tissue. Familial tendencies also play a role in more common cyst types. If your parents were prone to skin cysts, you’re more likely to develop them too.
How Cysts Differ From Abscesses
Because they can look and feel similar, cysts are often confused with abscesses. The distinction matters because the causes and treatment differ. A cyst is a closed sac surrounded by a distinct membrane, and its contents are usually fluid or semi-solid material like keratin. It’s typically painless unless it becomes inflamed. An abscess, by contrast, is a pocket of pus created by a bacterial or fungal infection. It tends to be red, warm, swollen, and painful from the start. Cysts can become infected and turn into abscesses, but an abscess is not a cyst by default.
Solid lumps that don’t contain fluid are generally classified as tumors or nodules, not cysts. Most cysts are benign, meaning they aren’t cancerous. Imaging like ultrasound can usually distinguish a fluid-filled cyst from a solid mass, which is why doctors often order imaging when a new lump appears rather than relying on feel alone.