Drainage of pus is a typical consequence of a bacterial infection. Bacterial infection can either be localized, such as an infection of a small cut or wound in the skin, or it may spread to involve larger areas of the body. A number of different infections can cause small bumps or nodules that may drain pus. Infections of the genital tract or urinary tract may cause drainage of pus in the urine or vagina in women. When bacteria enter the bloodstream, a potentially life-threatening condition known as sepsis ("blood poisoning") is the result. In addition to drainage of pus, other signs of a bacterial infection include swelling, warmth to touch, redness, and pain or sensitivity of the involved area. Bacterial infections are treated with antibiotics.

Source: http://www.rxlist.com