Acne Vulgaris is an inflammatory disease of the skin, caused by changes in the pilosebaceous units (skin structures consisting of a hair follicle and its associated sebaceous gland). Acne lesions are commonly referred to as pimples, spots or "zits". Acne develops as a result of blockages in follicles. Formation of a plug of keratin (part of the skin structure) and sebum (oil in the skin) is the earliest change. Enlargement of sebaceous glands and an increase in sebum production occur with increased androgen (horomone) production at puberty. The skin pore may enlarge to form an open comedo (blackhead) or closed comedo (whitehead). In these conditions the naturally occurring bacteria Propionibacterium acnes can cause inflammation, leading to inflammatory lesions (papules, infected pustules, or nodules) in the skin around the pore, which results in redness and may result in scarring or skin colour changes. Acne is most common during adolescence, affecting more than 85% of teenagers, and frequently continues into adulthood. For most people, acne diminishes over time and tends to disappear, or at least decrease, after one reaches his or her early twenties. 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. The most common form of acne is known as "acne vulgaris", meaning "common acne." Many teenagers get this type of acne. The face and upper neck are the most commonly affected, but the chest, back and shoulders may have acne as well. Aside from scarring, its main effects are psychological, such as reduced self-esteem and, according to at least one study, depression or suicide. Early and aggressive treatment is therefore advocated to lessen the overall impact to individuals.