What causes acne to form?

Acne is an infection of the skin, caused by changes in the sebaceous (oil) glands due to an increase in testosterone experienced at the on-start of puberty (by both males & females). The most common form of acne is called acne vulgaris. Oils from the glands combine with dead skin cells to block hair follicles. Under the blocked pore, propionibacterium acne's bacteria can grow very quickly resulting in pimples, pustules, redness (inflammation), blackheads, and uneven skin texture. The psychological effect of this skin reaction is often low self-esteem or even depression.