Acne is a disease. It is not caused by poor hygiene or eating chocolate. It occurs because of abnormal functioning of the pores in the skin.
Pores are composed of hair follicles and oil glands that normally lubricate the skin to keep it healthy. In acne, the oil glands are stimulated by hormones called androgens inside our bodies causing them to become over-active. During puberty, these hormones begin to stimulate the oil producing sebaceous glands in the pores. This causes increased production of oil known as sebum as well as stickiness of the walls of the pores.
Blackheads and whiteheads are simply pores that have become clogged with sebum, dead skin cells and trapped debris. Sometimes, bacteria called P. acnes that normally live in the pores take advantage of the very rich, oily environment and begin to grow. The growth of bacteria can cause the pore to become inflamed, forming a red, painful pimple. Sometimes, pimples grow so large that they fill with pus and can leave acne scars behind when they heal.