A pimple is a small pustule or papule. Pimples develop when sebaceous glands, or oil glands, become clogged and infected, leading to swollen, red lesions filled with pus.
During puberty, hormone production changes. This can cause the sebaceous glands, located at the base of hair follicles, to become overactive. As a result, pimples are most likely to occur during the teenage years and around menstruation, for women.
Pimples most often affect the face, back, chest, and shoulders. This is because there are many sebaceous glands in these areas of skin.
Acne vulgaris, the main cause of pimples, affects over 80 percent of teenagers. After the age of 25 years, it affects 3 percent of men and 12 percent of women.