Acne is caused by a plug of skin cells obstructing the flow of Sebum (an oily substance secreted by the sebaceous glands) at the neck of the hair follicles. When sebum becomes trapped in a follicle, bacteria multiply and the follicle becomes inflamed, taking on the appearance of a spot.
Acne can be a hereditary condition, and often manifests in both males and females during hormonal changes. The condition can be aggravated by some drugs, including steroids, barbiturates, and drugs used in the treatment of other health conditions such as epilepsy.
Acne occurs mainly on the face, in the center of the chest, on the upper back, on the shoulders and around the neck. The most common acne spots are open comedones (blackheads), closed comedones (whiteheads), papules, pustules, nodules (firm swellings below the skin) and cysts (larger, fluid-filled swellings in the skin).