Acne is a disease that affects the skin’s oil glands. 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. When the follicle of a skin gland clogs up, a pimple grows.

Most pimples are found on the face, neck, back, chest, and shoulders. Acne is not a serious health threat but, it can cause scars.

Source: NIAMS