Acne vulgaris its physiological, it depends on the harmonal variations and how  we responds to this variations depends on the heriditary. It occurs mainly on the areas where the sebeous glands distribution is rich like face (cheeks,fore head, on the chin..especially centrofacial)upper chest &back. Harmonal variation depends on the stress level,lack of sleep, menstrual cyles(in females)..etc Depending up on the grade of acne it will treated to prevent complications like post acne scarring & pigmentation.