pus, thick white or yellowish fluid that forms in areas of infection such as wounds and abscesses. It is constituted of decomposed body tissue, bacteria (or other micro-organisms that cause the infection), and certain white blood cells. These white cells form one of the defense mechanisms of the body. Known as phagocytes, they rush to the area of infection and engulf the invading bacteria in a process called phagocytosis. Many white cells themselves succumb in the process and become one of the constituents of pus.