Oil bacteria hormone. When oil glands of the face, shoulders, chest & back are matured by sex hormones, & overproduce oil, bacteria rapidly proliferate causing an inflammatory reaction. This then produces a commedoe or pimple. Some people inherit a much more vigorous reaction & get large, cystic, scarring lesions. Treatment is directed at decreasing oil & bacteria.
Really annoying! Seriously, acne is plugging of the oil glands, followed by inflammation and infection of the trapped oil by native bacteria. The process is influenced by hormones, stress, weather conditions, and other factors that have not been figured out yet.