Bacteria. Excess sebum clogs the openings to hair follicles -- especially those on the face, neck, chest, and back. Bacteria grow in these clogged follicles. This makes blackheads or whiteheads, also known as ''comedones,'' form on the skin's surface. Sometimes, this clogging causes the follicle wall to break under the pressure of this buildup. When this happens, sebum leaks into nearby tissues and forms a pustule or a papule -- this is called inflammatory acne. Larger, tender pustules are called nodules.
Acne (acne vulgaris, common acne) is a disease of the hair follicles of the face, chest, and back that affects almost all teenagers during puberty -- the only exception being members of a few primitive Neolithic tribes living in isolation. It is not caused by bacteria, although bacteria play a role in its development. It is not unusual for some women to develop acne in their mid- to late-20s.