They show up when you have some sort of infection and your body is trying to fight it off with white blood cells. That can result in a mixture of infected fluid and dead white blood cells (pus). When pus builds up under your skin or in a pore, it can cause a pustule.

Where did pustule come from?

Pustules may form when your skin becomes inflamed as a result of an allergic reaction to food, environmental allergens, or poisonous insect bites. However, the most common cause of pustules is acne. Acne develops when the pores of your skin become clogged with oil and dead skin cells.

Will a pustule go away?

Pustules are irritating but otherwise harmless, and they usually go away on their own. People can often prevent them or reduce their severity by keeping pustule-prone areas clean and oil free.