While adults sometimes have acne, nearly 100% of the population suffers breakouts at some point in their teenage years. Like the food at the school cafeteria, it is just an unpleasant part of growing up!
Propionibacterium acnes is the skin bacteria significantly responsible for the condition. The increased production of testosterone during puberty (which occurs in both boys and girls) causes the sebaceous glands to increase production of oily sebum. Normally, the oil just seeps out, keeping the skin lubricated and helping to remove dead skin cells. But when the flow increases, pores get clogged.
Since P. acnes loves sebum, it multiplies rapidly in skin pores, causing redness and swelling and a variety of lesions ranging from: open comedones (blackheads); closed comedones (whiteheads); papules (solid bumps); pustules (bumps containing pus); macules (flat red splotches); nodules (deeper and inflamed papules); and cysts (angry, deep-layer lesions that can cause scarring).
What can you do about acne? Creams with benzoyl peroxide can help in many cases. But if P. acnes won't take the hint, don't fool around – see a dermatologist.