Pus is a whitish-yellow, yellow, or brown-yellow protein-rich fluid called liquor puris that accumulates at the site of an infection.It consists of a buildup of dead, white blood cells that form when the body's immune system responds to the infection.When the buildup is on or near the surface of the skin, it is called a pustule or pimple. An accumulation of pus in an enclosed tissue space is called an abscess.