Where a pimple starts
A pimple can form for two or three weeks before you even know it exists, as it forms deep beneath the skin in the sebaceous gland beneath the hair follicle.
The pores begin to clog
Acne begins when the body produces excess androgen hormones that cause the sebaceous glands to make excess sebum. Sebum normally is produced to protect and keep the skin soft, but excess sebum can be problematic. It is common for this to happen during puberty, pregnancy or the menstrual cycle, though it can happen at other times. The excess sebum along with dead skin cells plugs the hair follicle. Anyone can get acne, but women and boys and girls in puberty get it more often.
Bacteria Begins to Form
Unwanted bacteria begin to form in the sebaceous glands because the hair follicle is blocked by hair, dead skin and excess sebum and cannot excrete it. Pus continues to build up as the infection grows. This causes the body to react by attacking the bacteria with white blood cells. This causes redness, inflammation and sometimes pain. It is at this point that we become aware of the pimple. We see the inflammation more than the clogged pore.
The pimple grows
Without treatments, a pimple can continue to form over the skin and send bacteria to surrounding tissues, forming a papule. This can turn into a pustule, which is essentially a cyst. Attempting to pop a pimple may also force it into neighboring tissues, so it is not recommended.
The pimple is healed
Eventually due to time, cleansing, the skin or applying medication, the hair follicle releases the clog and the dead skin cells, bacteria, and sebum spill from the pimple. The red blood cells can also cause the pimple to shrink and dry up without ever excreting any or much sebum and without causing any scar or injury to the skin.