While sometimes it seems like pimples appear overnight, the development of an acne breakout is actually an extended process that begins at the cellular level. Each follicle is connected to sebaceous glands; these glands secrete an oil substance know as sebum that must go up to the surface of the skin where it lubricates your hair and skin.

When your body produces an excess amount of sebum and dead skin cells, the two can build up in the hair follicles and together form a soft plug. The obstruction injures the linning of the sebum canals and the body reacts with an inflammatory breakout and swelling, causing whiteheads. At times the plug is large enough that the material blocking the pore can be seen, a phenomenon commonly known as blackheads.

If the obstruction causes a rupture in the wall of the pore, the dead cells of the skin and the lubricant oil can block the sebum canals. Bacteria that is normally beneficial can start proliferating and forming small infected areas called pustules (also known as pimples). The lesions may be followed by an immoderate inflammatory reaction which destroys not only the bacteria and the sebum that blocking the pores but also your own skin cells. This tissues damage is waht causes acne scarring. If the infected areas are deep within skin, they can increase in size until forming a firm and painful nodular cyst.

A strong increase in sebum production occurs a few hours after birth; this peaks during the first week and slowly subsides thereafter. A new rise takes as early as age 9 with the onset of adrenal androgen secretion, and the signs of puberty in boys or girls, including pubic hair, axillary hair, and adult body odor. This period of heavy sebum production continues up to age 17 years, when the adult level is reached.

Therefore, almost all teenagers get acne at one time or another since during puberty the body
produces a lot of sebum and there is an excessive shedding of follicle cells. This mixture of oil and
dead cells becomes a breeding ground for bacteria. By the age of 20, most people's acne has cleared up, however, it may still persist throughout the adult years, and some people even experience acne for the first time when they are adults.