Pimples or acne often begin in your teenage years. They start to appear because your skin makes too much oil, due to a hormone imbalance. The sebaceous glands of people with acne are especially sensitive to normal blood levels of a hormone called testosterone, found naturally in both men and women. Normally the pimples appear on your face, chest and back, as these areas contain many oil glands. The pores of a skin that has too much oil get blocked, and bacteria (germs which live in everyone's skin and generally cause no problems) grow more easily there. It is the bacteria what cause red, swollen pimples.