Acne is caused by the production of excess sebum - the greasy substance that moisturises the hair follicle and the skin. Dead skin cells can mix with excess sebum and block the pores, causing spots. The normal bacteria that live on the surface of your skin can multiply inside the blocked pores, leading to pus-filled papules.
Acne mainly affects the face but can also affect the neck, back and chest. It is very common amongst teenagers and usually resolves by itself over a few years, however it can persist on into later life in a small minority of people. As acne is visible to others, it can be very distressing and have a negative impact on your quality of life.