Our skin produces sebum oil all the time which should find its way out through the follicle ducts in our skin. When these ducts get blocked due to some reason, the sebum is trapped resulting in the skin getting inflamed. If a certain bacterium P. Acnes, which resides in the skin, infects that skin, the skin will get acne.

Source: Health and Fitness: Acne Articles from EzineArticles.com