Acne is an infection of the skin, caused by changes in the sebaceous glands which usually happen during puberty. Acne can be treated with a solution of Bubotuber pus.
Acne is also a part of a Mandrake's maturing cycle; once their acne has cleared up they are ready for repotting and after that reach maturity.
Eloise Midgen was known to have a bad case of acne  and many of her colleagues at Hogwarts School used her as the standard against which really bad acne was to be measured.
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