It starts when greasy secretions from the skin's sebaceous glands (oil glands) plug the tiny openings for hair follicles (plugged pores). If the openings are large, the clogs take the form of blackheads: small, flat spots with dark centers. If the openings stay small, the clogs take the form of whiteheads: small, flesh-colored bumps. Both types of plugged pores can develop into swollen, tender inflammations or pimples or deeper lumps or nodules. Nodules associated with severe cases of acne (cystic acne) are firm swellings below the skin's surface that become inflamed, tender, and sometimes infected.
Diet. Studies indicate that certain dietary factors, including skim milk and carbohydrate-rich foods — such as bread, bagels and chips — may worsen acne. Chocolate has long been suspected of making acne worse. A small study of 14 men with acne showed that eating chocolate was related to a worsening of symptoms. Further study is needed to examine why this happens and whether people with acne would benefit from following specific dietary restrictions.