Acne. Illustration of a section through the skin showing a pustule (pus-filled blister) forming in a hair follicle (centre) in the skin. The follicle has become blocked with sebum, an oily substance produced by the sebaceous gland (yellow). This allows bacteria present to multiply, causing inflammation and a pimple or pustule. The most common form of acne is acne vulgaris, which mainly affects adolescents. Treatment is with antibiotics.