Acne is an inflammatory disease of the skin, caused by changes in the pilosebaseous units (skin structures consisting of a hair follicle and its associated sebaceous gland). 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, shoulders and upper arms are especially affected.

The typical acne lesions are: comedones, papules, pustules and cysts. More inflamed rashes take the form of pus-filled, or reddish bumps, even boil-like tender swellings. After resolution of the lesions, prominent unsightly scars may remain.

The condition 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 the third decade of life. 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.

Exactly why some people get acne and some do not is not fully known. It is known to be partly hereditary. Several factors are known to be linked to acne:

Stress
Hormonal activity
Hyperactive sebaceous glands
Accumulation of dead skin cells
Bacteria in the pores
Skin irritation or scratching of any sort
Anabolic steroids

Any medication containing halogens (iodides, chlorides, bromides), lithium, barbiturates, or androgens. Exposure to high levels of chlorine compounds, particularly chlorinated dioxins, can cause severe, long-lasting acne, known as Chloracne

Traditionally, attention has focused mostly on over-production of sebum as the main contributing factor of acne. More recently, more attention has been given to narrowing of the follicle channel as a second main contributing factor. Abnormal shedding of the cells lining the follicle, abnormal cell binding ("hyperkeratinization") within the follicle, and water retention in the skin (swelling the skin and so pressing the follicles shut) have all been put forward as mechanisms involved, but there does not appear to be much conclusive medical research on the subject.

Since the medical knowledge about acne is still relatively small, there are many misconceptions and rumours about what causes the condition:

Diet. Chocolate, chips and sugar, among others, have not been shown to affect acne. This means that the scientific studies done to date did not find a big difference between acne in two groups of people, one group eating the food in question and one group avoiding it. However, one recent study, based on a survey of 47,335 women, did find a positive association between milk consumption and acne. The researchers hypothesize that the association may be caused by the hormones in milk. Seafood, on the other hand, may contain relatively high levels of iodine, but probably not enough to cause an acne outbreak. Still, people who are prone to acne may want to avoid excessive consumption of foods high in iodine.

Deficient personal hygiene. Acne is not caused by dirt. This misconception probably comes from the fact that acne basically involves skin infections. In fact the blockages that cause acne usually occur deep within the narrow follicle channel, where it is usually impossible to wash them away, from the cells and sebum created there by your body. The bacteria involved are exactly the same bacteria that everyone has on their skin. It is useful to clean your skin, but doing so will not prevent acne. Anything beyond very gentle cleansing can actually worsen existing lesions and even encourage new ones by damaging or overdrying skin.