Cysts are bumps that contain fluids, gas or air, and can occur on the skin, or in many different parts of the body. Most are benign, but there are some that require medical attention, as they can be dangerous. In general, a cyst may be dangerous if it affects a major organ or the breasts, if it grows particularly large, or if it ruptures.
Some of the most common locations for cysts to form in the body are on the ovaries, in the kidneys, on the esophagus, and in the pancreas. These are generally not dangerous, but they can affect the function of these organs. Most of the time a doctor will test it to make sure it's not cancerous. If it's benign — as is often the case — the doctor will keep it under careful observation. If it looks like it's going to rupture, then it will be removed.
Even in situations where a cyst isn't dangerous, it can cause problems. For instance, these structures in the esophagus are generally benign, but they can affect speaking and singing. These often have to be removed, especially for professional speakers or singers. Likewise, many people have small cysts in their kidneys and joints that typically aren't dangerous, but can cause problems if they get too large. Skin cysts are also usually not dangerous, but may be removed if they get infected or for cosmetic reasons.
One situation in which these bumps are almost always dangerous is when they form in the lungs and pancreas, which happens in those with cystic fibrosis. They can severely inhibit breathing and make people more vulnerable to lung infections. There is no cure for cystic fibrosis, but the symptoms can be treated. Ovarian bumps can be dangerous too if they get very large or rupture, as a ruptured cyst can impair fertility. Any lump in the breast that might be a cyst should be tested immediately, since it could be a cancerous lesion.
A cyst that is not dangerous in itself can still be a sign of a dangerous condition. For example, exposure to giardia often results in small cysts being passed in the stool of those infected. This parasite can actually pass on to other people through contact with the passed cyst, which protect and keep alive the giardia.