Acne is a disease unique to humans and is associated with sebaceous glands that are found in a high density on the scalp, forehead and face. Despite being a near universal problem in adolescence, the reason why such troublesome sebaceous glands exist at all is not well understood. Some interesting theories have been postulated including roles for skin maintenance, immunological function and perhaps even pheromones, but pre-pubertal skin which has sebaceous glands that are largely inactive, is healthy.