Pus is a thick fluid that accumulates at the site of an infection. It is a whitish-yellow, yellow, green, or brown-yellow protein-rich fluid called liquor puris. Pus consists of dead white blood cells, dead tissues, and dead bacteria. It is a natural result of the body fighting infection. Pus can be yellow, green, or brown, and may in some cases have a foul odor. When the body detects an infection, it sends white blood cells, called neutrophils, to destroy the bacteria. During this process, some of the neutrophils and tissue surrounding the infected area will die, and pus is an accumulation of this dead material.

Pus can 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. Pus generally forms in an abscess, which is a cavity or space created by the breakdown of tissue. Abscesses can form on the skin’s surface or inside the body.

Fast facts about pus include:

• Pus is a natural result of the body fighting infection.
• Pus can be yellow, green, or brown, and may in some cases have a foul odor.
• If pus appears after surgery, contact a doctor immediately.
• Smaller buildups of pus can be self-managed at home.
• Larger or less accessible buildups of pus may require surgical intervention and the application of a drainage channel.

In almost all cases when there is a collection of pus in the body, a clinician will try to create an opening to drain it. This principle has been distilled into the famous Latin aphorism "Ubi pus, ibi evacua" ("Where there is pus, evacuate it").