Acne is more than a medical phenomenon
If you think that acne is purely medical and an internal process, then here is something new for you. When skin cells plug skin pores, they cause whiteheads and when it aligns with pigment then you have blackheads. Now, a pimple is an oil gland that is infected with bacteria (that's why you would observe that pimples are moreacne guide common when you are exposed to germy conditions) resulting in a red spot with pus formation.