A pustule is a bulging patch of skin that's full of a yellowish fluid called pus. It's basically a big pimple. Several conditions, ranging from something as common as acne to the once-deadly disease smallpox, can cause pustules.
They show up when you have some sort of infection 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.
