Boils are common bacterial infections that occur deep inside the skin. They are also referred to as skin abscesses. Initially, the area (where the boil develops) turns reddish and inflamed. Within a few days, it turns more tender, firm, and hard. By this time, white blood cells (WBCs) act against the infection causing bacteria, which lead to the formation of a pus-filled abscess. At this stage, the boil is soft to touch and raised from the surrounding skin area with a distinct head.