An abscess is a collection of pus underneath the skin or inside your body. Abscesses usually form because of an infection or because a foreign object becomes trapped in your body. The vast majority of them are caused by infections. Inside, they are full of pus, bacteria and debris. Abscesses develop as a result of the interaction between invading micro-organisms and host immune defences. Abscesses can form in almost any part of the body. The skin, under the skin, and the teeth are the most common sites. Abscesses may be caused by bacteria, parasites, and foreign substances. Abscesses in the skin are easy to see. They are red, raised, and painful. Abscesses in other areas of the body may not be seen, but they may cause organ damage.