An abcess is a localized collection of pus in any part of the body, caused by an infection. Abscesses occur when an area of tissue becomes infected and the body is able to "wall off" the infection to keep it from spreading. White blood cells, the body's defense against some types of infection, migrate through the walls of the blood vessels into the area of the infection and collect within the damaged tissue. During this process "pus" forms, which is an accumulation of fluid, living and dead white blood cells, dead tissue, and bacteria or other foreign invaders or materials. Abscesses can form in almost every part of the body and may be caused by infectious organisms, parasites, and foreign materials. Abscesses in the skin are readily visible and are red, raised, and painful. Abscesses in other areas of the body may not be obvious, but if they involve vital organs they may cause significant damage.