Acne is the result of a blockage or infection of the pilosebaceous unit (composed of a hair follicle, arrector pili muscle, and sebaceous gland).
In simple words, acne happens when the hair follicles become clogged with oil and dead skin cells, creating an anaerobic environment (which means there is not any oxygen within the pilosebaceous unit) so the C. Acne occurs and proliferates.
The skin responds to this infection by creating inflammation in the form of red, sore, swollen pimples, pustules or pumps.