A cyst is a closed sac having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess not a cyst. Once formed, the cyst could go away by itself or will have to be removed using surgery.
I've got cysts on my ovaries -- discovered when I was 19, and lead directly to my PKD diagnosis.
I've got cysts in my kidneys -- polycystic kidney disease is aptly named.
I've got cysts in my liver -- a very common side-effect of PKD.
Aaaaaaaaaaaand, I've got cysts in my breasts -- necessitating an early mammogram two years ago at age 39, and prompting a recall this month on this year's mammogram. Apparently the cysts have grown or moved, and the radiologist wants an ultrasound to get a better look.
I was beginning to feel like a big old pus-bag until I checked Wiki and learned that while cysts may be filled with fluid, they are not filled with PUS. If they're filled with pus, they're called something else.
Isn't that comforting?
(I go in for the ultrasound on Friday. I am incapable of writing engagingly about having my breasts mashed. For a much better essay on the topic than I am capable of, go visit Mir at Woulda Coulda Shoulda. She's pretty.)