Acne forms near the opening of the sebaceous glands that have hair follicle in them. In normal circumstances, the cells that form the wall of the gland should exfoliate from the pore opening. In certain circumstances these cells become dysfunctional and instead of exfoliating normally, block the pore opening.
In the meantime, the glands keep producing sebum and if hormonal disturbance is present the glands produce sebum at a higher rate. This sebum keeps filling the gland sack and the bacteria called Proprionobacterium acnes (P. acnes) thrive in the atmosphere that is full of sebum and no air. As the bacteria flourishes, the gland gets infected.
At this stage when the bacteria are flourishing and causing inflammation, the gland is producing more sebum and the pore is blocked, the gland becomes very infective and full of dead cells, bacteria, pus and dead skin. This is what we call as a papule or a cyst. The acne grows through different stages and this is the last stage after which the gland ruptures.
Thus acne is not a single stage disease but many causes join together to progressively increase the intensity of acne formation. Teenagers get more acne...