An abscess is a pocket of infected fluid with a thick wall around it. Skin or soft tissue abscesses occur when bacteria get into tissue below the outer layer of skin. Often abscesses occur when hair follicles or sweat glands get infected or after minor scrapes or puncture wounds.

Most of the time an abscess forms when bacteria enter a break in the skin. For example, an abscess might develop when a hair becomes ingrown or when the skin is scratched or poked with something sharp. As more bacteria grow (multiply) in the skin or soft tissues, the body responds by forming a wall around the area to keep the bacteria from spreading.

The bacteria in this pocket continue to multiply and as they do, the pocket becomes more swollen. Some of the bacteria may get through the wall and cause an infection of the tissues around the abscess or even get into the blood and infect other parts of the body.
 

Sometimes, these abscesses are caused by a bacteria (called Staph), that is resistant to certain antibiotics. This is known as MRSA.