An abscess is pus that collects in pockets in many different places around the outside of the body. It is usually a thick fluid that contains lots of white blood cells, dead tissue, and bacteria. The pus may be yellow or green, have a bad smell, or appear to be thick. It is usually the result of a small infection, which causes the immune system to set off white blood cells and other antibodies to combat the infection. Sometimes, this causes tissue to die, which forms a cavity. The cavity becomes larger and larger and sometimes the pus from the infection cannot escape.