Acne is a skin disease involving the oil glands at the base of hair follicles. Whilst not necessarily dangerous, acne can leave scars. Common causes of acne include genetics, the menstrual cycle, anxiety and stress, hot and humid climates and using oil-based makeup. Skin have pores that connect to oil glands under the skin. These glands produce an oily liquid called Sebum which carries dead skins through follicles to the surface of the skin.