Cysts form when fluid, air, or semi-solid material gets trapped in a pocket of tissue, creating a sac-like structure. The specific cause depends on where the cyst develops and what type it is, but the underlying triggers fall into a handful of categories: blocked ducts or glands, hormonal shifts, physical injury, infections, and genetics. Most cysts are benign, and many resolve on their own without treatment.
Blocked Glands and Trapped Protein
The most common cysts people notice are the ones that form just under the skin. These typically develop when something blocks a hair follicle, oil gland, or sweat duct, trapping material that would normally drain to the surface. Pilar cysts, which show up most often on the scalp, grow directly from hair follicles and fill with keratin, the same protein that makes up your hair and nails. Epidermoid cysts work similarly: skin cells that would normally shed get pushed deeper into the skin instead, where they accumulate inside a small pocket and slowly expand.
Sebaceous cysts form from the glands that produce the oily substance coating your skin. When the outlet of one of these glands gets plugged, the oil backs up and forms a firm, round lump. These blockages can happen randomly, but they’re more likely after minor skin injuries, acne, or irritation that damages the opening of a follicle or gland.
Hormonal Triggers and Ovarian Cysts
Ovarian cysts are extremely common. In screening studies, simple cysts were visible on ultrasound in 14% of women over 55 at their first scan, with roughly 8% developing a new cyst each year. In younger, menstruating women the rates are even higher, because the most frequent type of ovarian cyst is a direct byproduct of ovulation.
Each month, your ovaries grow small fluid-filled sacs called follicles that produce estrogen and progesterone. Normally a follicle ruptures to release an egg, then shrinks. A follicular cyst forms when that rupture doesn’t happen: the follicle stays intact and keeps growing. A corpus luteum cyst takes the opposite path. The follicle does release its egg, but the opening seals shut afterward, trapping fluid inside the now-empty sac.
Both types usually resolve within a few menstrual cycles. But hormonal imbalances can make them more frequent. Fertility medications that stimulate ovulation increase the risk, and polycystic ovary syndrome (PCOS) involves chronic hormonal disruption that leads to many small cysts on the ovaries. Endometriosis can also cause ovarian cysts called endometriomas, which form when tissue similar to the uterine lining attaches to an ovary and creates a blood-filled pocket.
Lifestyle Factors That Affect Risk
For ovarian cysts specifically, several modifiable factors influence how likely they are to form. Smoking raises the risk, particularly in women with a lower body mass index. Alcohol consumption, a diet high in processed foods and sugar, and physical inactivity are all associated with higher cyst prevalence. Vitamin D deficiency is also common in women who develop cysts frequently. On the other hand, regular exercise, a balanced diet, and oral contraceptive use all appear to lower the risk of functional ovarian cysts.
Joint Stress and Ganglion Cysts
Ganglion cysts are rubbery lumps that develop along tendons or joint capsules, most often on the wrist or hand. They’re filled with a thick, jelly-like fluid similar to the lubricant inside your joints. The exact mechanism isn’t fully understood, but joints or tendons that have been injured are significantly more likely to develop them. Repetitive stress on a joint, such as from gymnastics or manual labor, is a common pattern. The leading theory is that micro-damage to the joint capsule or tendon lining allows synovial fluid to leak out and pool in a small sac that gradually enlarges.
Physical Trauma
A hard impact can create the conditions for a cyst to form months or even years later. When a significant blow causes bleeding deep in the soft tissue, the resulting pool of blood (a hematoma) sometimes fails to reabsorb. Instead, the body walls it off with fibrous tissue, creating a chronic fluid-filled cavity. This happens at the boundary between the fat layer under your skin and the tougher connective tissue over your muscles, where crushing and shearing forces during the injury separate those layers.
The inflammatory response plays a role too. As red blood cells and clotting proteins inside the hematoma break down, they trigger ongoing inflammation that causes plasma to keep leaking from nearby blood vessels into the capsule, which makes the cyst grow rather than shrink. On a smaller scale, any minor injury to the skin, a cut, a scrape, or even a splinter, can push surface cells deeper into tissue, seeding what’s called an inclusion cyst.
Infections and Parasites
Certain infections cause cysts as a direct part of their life cycle. The most well-known example is cystic echinococcosis, caused by accidentally swallowing eggs from the Echinococcus granulosus tapeworm (typically through contaminated food or contact with infected dogs). The larvae migrate through the body and form slow-growing cysts, most often in the liver or lungs, though they can appear in virtually any organ. Because the cysts grow so slowly, infected people may have no symptoms for years.
Bacterial infections can also lead to cyst formation. An abscess, while not technically a cyst, can evolve into one if the body walls off the infection with fibrous tissue. Bartholin’s cysts, which form near the vaginal opening, develop when the ducts of the Bartholin’s glands become blocked, often after a bacterial infection causes swelling that seals the duct shut.
Genetic Conditions
Some people are genetically predisposed to developing cysts throughout their body. Polycystic kidney disease is one of the most common inherited cyst disorders. In the autosomal dominant form, mutations in specific genes disrupt the proteins that maintain normal kidney tube structure. Without proper protein function, kidney cells grow and divide in an uncontrolled way, and fluid accumulates in expanding pockets. Tiny molecules called microRNAs compound the problem by further suppressing the proteins meant to keep cell growth in check, accelerating cyst formation over time.
Von Hippel-Lindau syndrome is a rarer inherited condition caused by mutations in a single gene that normally acts as a brake on cell growth. When that gene malfunctions, cysts and tumors develop across multiple organs, including the kidneys, pancreas, reproductive tract, and central nervous system. Other genetic syndromes like Gardner syndrome and steatocystoma multiplex similarly cause clusters of cysts as a hallmark feature.
How Cysts Differ From Tumors
One reason people search for cyst causes is anxiety about cancer. The key distinction is that a true cyst is a fluid-filled sac, while a tumor is a solid mass of tissue. On ultrasound or CT, a cyst that appears uniform and entirely fluid-filled is almost always benign. If a cyst contains solid components, mixed in with the fluid, further evaluation is needed because that pattern can occasionally indicate something more serious. Most cysts, regardless of location, fall clearly into the benign category and either resolve on their own, remain stable, or are drained if they cause discomfort. Among ovarian cysts tracked over time, about a third disappeared within a year without any intervention, and more than half simply stayed the same size.