Common human skin condition that has many forms. The most common and most familiar is acne vulgaris, which occurs most commonly in adolescence when testosterone increases in both sexes. The pimples, pustules, blackheads, and whiteheads occur because of inflammatory responses by the skin within both the hair follicles and the accompanying sebaceous glands. Scarring and psychological distress are common. It often clears up in most people in their late thirties or early forties, though it may persist longer.