Pus is a whitish-yellow or yellow substance that can be found in regions of bacterial infection, including superficial infections, such as pimples. An accumulation of pus in an enclosed tissue space is known as an abscess.

Pus consists of macrophages (a type of white blood cell), debris, dead and dying cells and necrotic tissue complements, formed by the process of necrosis. Necrosis is caused by special enzymes, released by lysosomes.

When seen in a wound, pus indicates it is infected, and some say it should be cleaned with an antiseptic.