Acne is due to a blockage of oil glands. Oils glands or sebaceous glands are located in higher numbers on the face, back and shoulders, areas mainly affected by acne. Once the oil glands are blocked, a blackhead forms. Blackheads can evolve into pustules, or zits.
The severity of acne is dependent on genetics, hormonal factors, stress, and in some cases diet. If there is a family history of acne, this may increase your likelihood of developing acne.