No one factor causes acne. Acne happens when oil (sebaceous) glands are activated at puberty, stimulated by male hormones from the adrenal glands in both boys and girls. Oil is a natural substance which lubricates and protects the skin. Under certain circumstances, cells that are close to the surface block the openings of sebaceous glands and cause a buildup of oil underneath. This oil stimulates bacteria, which live in everyone's skin and generally cause no problems, to multiply and cause surrounding tissues to become inflamed.
If the inflammation is right near the surface, you get a pustule; if it's deeper, a papule (pimple); deeper still and it's a cyst. If the oil breaks though to the surface, the result is a "whitehead." If the oil becomes oxidized (that is, acted on by oxygen in the air), the oil changes from white to black, and the result is a "blackhead."
In occasional patients, contributing factors may be: