Very short answer: Acne is caused by clogged pores, which can get infected.
Some more details:
The sebaceous glands just under the surface of your skin produce oily stuff called sebum.
Sebum is in fact supposed to help keep your skin clean. It carries dead skin cells up through small canals – called follicles – to 'eject' that unwanted material on the skin's surface.
Your sebaceous glands grow and become active during adolescence. (That's one reason you had such awesome skin as a baby, and then it suddenly turned into your worst enemy when you hit the early teen years.)
Anyway sometimes a follicle gets blocked by dirt or dead skin. Sebum builds up underneath. Certain bacteria trapped in the plug start to cause infection. The infection causes redness and swelling. Congratulations: You've got acne. (We've all been there. See what Cor customers say about fighting acne.)
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