Acne is a pustular infection of the skin, caused by changes in the sebaceous glands. The most common form of acne is known as acne vulgaris, which means common acne. Excessive secretion of oils from the glands combine with naturally occurring dead skin cells to block the hair follicles. Oil secretions build up beneath the blocked pore, providing a perfect environment for the skin bacteria Propionibacterium acnes to multiply uncontrolled. In response, the skin inflames, producing the visible lesion. The face, chest, back and upper arms are especially exposed.

The infection is common in puberty as a result of an abnormal response to normal levels of the male hormone testosterone. The response for most people diminishes over time and acne thus tends to disappear, or at least decrease, after one reaches early adulthood. There is, however, no way to predict how long it will take for it to disappear entirely, and some individuals will continue to suffer from acne decades later, into their thirties and forties and even beyond.

Acne affects a large percentage of humans at some stage in life. Aside from scarring its main effects are psychological, such as reduced self-esteem and depression. Acne usually appears during adolescence, when people already tend to be at their most socially-insecure. For this reason acne should be treated if severe.