What are the causes of pimples?

The sebaceous glands, which produce sebum, exist inside the pores of our skin. The outer layers of our skin are being shed continuously.

Sometimes, dead skin cells are left behind and get stuck together by the sticky sebum, causing a blockage in the pore.

Pore blockage is more likely to occur during puberty (the process of physical changes by which a child's body becomes an adult body capable of reproduction). More sebum is produced by the sebaceous gland - as the pore is blocked, it accumulates behind the blockage.

This accumulated and blocked sebum has bacteria, including Propionibacterium acnes; this slow-growing bacterium is linked to acne. Propionibacterium acnes generally exists harmlessly on our skin - however, when the conditions are right, it can reproduce more rapidly and become a problem. The bacterium feeds off the sebum and produces a substance that causes an immune response, leading to inflammation of the skin and spots.
 "Good" and "bad" bacteria determine severity and frequency of pimples:
 he skin of people who are prone to acne are especially sensitive to normal blood levels of testosterone - a natural hormone found in both males and females. In such people the testosterone can make the sebaceous glands produce too much sebum, making the clogging up of dead skin cells more likely, which in turn increases the probability of blocking the pores, etc.

You cannot catch pimples from another person; they are not infectious.