Cystic acne is a type of inflammatory acne that causes painful, pus-filled pimples to form deep under the skin. Acne occurs when oil and dead skin cells clog skin pores. With cystic acne, bacteria also gets into the pores, causing swelling or inflammation. Cystic acne is the most severe type of acne. If ignored, cysts can take anywhere from 1-4 weeks to go away by themselves. Although your body will eventually deal with the inflammation, some cysts can persist for extended periods of time and often leave behind scars.