Abscesses are typically caused by either an inflammatory reaction to an infectious process (bacteria or parasite) or less commonly, to a foreign substance within the body (a needle or a splinter, for example).
Abscesses may develop because of obstructed oil (sebaceous) or sweat glands, inflammation of hair follicles, or from minor breaks and punctures of the skin. The infectious organisms or foreign material cause an inflammatory response in the body, which triggers the body's immune system to form a cavity or capsule to contain the infection and prevent it from spreading to other parts of the body. The interior of the abscess liquefies, and pus develops (which contains dead cells, bacteria, and other debris). This area then begins to expand, creating increasing tension and inflammation of the overlying skin.
People with weakened immune systems can develop abscesses more often because their body's ability to fight infection is decreased. Individuals with any of the following conditions are at higher risk for developing abscesses: