Acne is a skin condition that occurs when your hair follicles become plugged with oil and dead skin cells. The plugged hair follicle gradually enlarges. As the follicle enlarges, the wall may rupture, allowing bacteria access into the deeper layers of the skin, ultimately producing inflammation and sometimes infection. This then often causes whiteheads, blackheads, pimples or cysts, and usually appears on the face, forehead, chest, upper back and shoulders. Acne is most common among teenagers, though it affects people of all ages. No one factor causes acne.