A bubo is a swelling under the skin caused by an infected lymph gland. They are typically located in the groin, armpit, or alongside the throat. Buboes appear as tender lumps about the size of a chicken egg and, depending on their location, can cause significant discomfort and limit the use of the limbs. The swelling is caused by the inflammation of the lymph nodes in response to a bacterial infection, such as gonorrhea, syphilis, tuberculosis, or, most famously, bubonic plague.
The lymphatic system is a vital part of the immune system, and serves to filter harmful bacteria from the bloodstream. When a significant bacterial population builds up in a lymph node, it responds by increasing blood flow to the node, the body temperature, and the levels of blood-borne antibodies within the node to destroy the foreign bacteria. This also causes the lymph node to become enlarged and often tender to the touch. A bubo can occur when a lymph node builds up a population of infectious bacteria that is resistant to the adverse conditions created by inflammation. This leads to an overreaction of the inflammation response and excessive swelling of the lymph node, without significantly impacting the infectious bacteria.
Since a bubo is generally one of the most visible and unpleasant early symptoms of a serious infection, it is a common target for direct treatment. Bubonic plague victims often had their buboes lanced or drained, which resulted in the spread of potentially infectious blood and pus, as well as a potential source of secondary infection for the patient. Draining a bubo does not significantly reduce the levels of infectious bacteria within the body, and is not a recommended course of treatment. Buboes generally shrink and vanish as the infection is defeated, either through the effects of the patient's own immune system or through the use of antibiotics.
The appearance of a bubo is a strong indicator of a serious bacterial infection, and should be examined by a medical professional as soon as possible. By taking samples of blood and pus from the bubo, and testing for bacterial infection, the exact nature of the disease can be determined. While the bubonic plague is extremely rare in the modern age, significant outbreaks of the disease continued well into the Twentieth Century, and isolated cases in the United States have been reported as recently as 2010. Buboes caused by infections such as gonorrhea, tuberculosis, and syphilis are much more common.