Acne Causes
It is important to realize that acne results from a combination of
factors. Briefly reviewing how acne is caused will help one
understand how to treat acne and how to cure acne. Also, it is
important to understand what NOT to do so one doesn't make acne
worse.

Located primarily on one's face, back and chest are glands known as
sebaceous glands. These glands are responsible for The production
of sebum, your skin's natural oil. These glands are located in
specialized follicles known as sebaceous follicle, or pores in simple
terms.  Acne results from a cascade of events. The first is the pore
becomes plugged due to excessive amounts of sebum.  Next dead
skin gets trapped in the pores.  This results in the non-inflammatory
lesions of acne "blackheads" and "whiteheads."  

The next factor involved is The bacteria P. Acnes. The excessive
sebum combines with dead skin to create a nice environment for P.
Acnes to grow. One's immune system recognizes this and tries to kill
P. Acnes. The resulting battle of your immune system against The
bacteria results in The red, painful inflammatory lesions known as
papules, pustules, and nodules.
Acne101.org - "A Doctor Compiled Directory On All Aspects of Acne and Its Treatment"