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Pus is a whitish-yellow, yellow, or yellow-brown exudate produced by vertebrates during inflammatory pyogenic bacterial infections. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule or pimple. Pus is produced from the dead and living cells that travel into the intercellular spaces around the affected cells.

Something that creates pus is called suppurative, pyogenic, or purulent. If it creates mucus as well as pus, it is called mucopurulent. The most common agents that induce pus formation are bacteria, such as Staphylococcus aureus.

Pus consists of a thin, protein-rich fluid, known as liquor puris, and dead cells, which are part of the body's immune response. Neutrophils are produced in the bone marrow and released into the blood. When the need to fight infection arises, they move to the site of infection by a process known as chemotaxis, usually triggered by cytokines released from macrophages that sense invading organisms. At the site of infection, the neutrophils engulf and kill bacteria. Eventually, the neutrophils die, and are then phagocytosed by macrophages, which break them down further. Pus is viscous material composed of these dead neutrophils.

Neutrophils are the most abundant type of leukocyte in human blood, making up anywhere between 40% and 75% of leukocytes.

When seen in a wound or dry skin, pus indicates the area is infected and should be cleaned with antiseptic.

Despite normally being of a whitish-yellow hue, changes in the color of pus can be observed under certain circumstances. Pus is sometimes green because of the presence of myeloperoxidase, an intensely green antibacterial protein produced by some types of white blood cells. Blue-green pus is found in certain infections of Pseudomonas aeruginosa as a result of the pyocyanin bacterial pigment it produces; amoebic abscesses of the liver produce brownish pus. Pus can also have a foul odor.

In almost all cases when there is a collection of pus anywhere in the body, the clinician will try to create an opening for it to evacuate - this principle has been distilled into the famous Latin aphorism "Ubi pus, ibi evacua!"

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