Acne is a disorder that affects the skin’s oil glands and hair follicles. The small holes in your skin (pores) connect to oil glands under the skin. These glands make an oily substance called sebum. The pores connect to the glands by a canal called a follicle. Inside the follicles, oil carries dead skin cells to the surface of the skin. A thin hair also grows through the follicle and out to the skin. Sometimes, the hair, sebum, and skin cells clump together into a plug. The bacteria in the plug cause swelling. Then when the plug starts to break down, a pimple grows.