Boils are round, red, painful, pus-filled bumps on the skin that are caused by a bacteria infection. When a staphylococcus aureus (staph) infection infects a hair follicle it develops deep in the root of the follicle and works its way to the skin's surface, forming a boil. Most people will notice mild pain, itching, and swelling at first. Usually within a day the boil becomes red and begins to fill with puss and becomes increasingly painful.

When a boil first begins to develop, it is small, red, and tender. After a few days the boil begins to collect pus, which is actually a mass of white blood cells that your body's immune system has sent to fight the infection. The center of the boil begins to take on a whitish color.