Acne occurs when the oil (or sebaceous) glands under hair follicles—often on your face, neck, chest, or back—get plugged up, inflamed, or infected. Frequently the glands simply secrete too much of a thick, waxy lubricant called sebum that keeps your skin healthy. Sebum can block the pores and create a perfect breeding ground for bacteria. Follicles then get red and inflamed. In mild cases of acne (referred to as acne vulgaris), whiteheads and blackheads as well as some raised red blemishes appear. In more severe cases (cystic acne), pus-filled cysts and deep nodules form around the inflamed follicles, causing unsightly bumps and tenderness, which are often painful.