The visible signs of acne are actually caused by issues that develop underneath the skin.
In this layer of the skin there are tiny structures called sebaceous glands that produce an oily substance called sebum that lubricates the surface of your skin and hair.
Sebum usually passes through the hair follicle out into the surface of the skin. When the follicle becomes blocked, however, the sebum begins to build up inside, leading to the formation of pimples and spots on the surface of the skin.
This blockage can happen for a variety of reasons: most frequently, it is caused by an excess of sebum being produced, or dead skin cells or makeup blocking the exit point onto the surface of the skin.
Once the blockage has been created, sebum continues to be produced, making the issue worse.