A bubo is an enlarged lymph node (“swollen gland”) that is tender and painful, particularly in the groin and armpit (the axilla). A feature of a number of infectious diseases including gonorrhea, syphilis, tuberculosis, and the plague. Hence, the bubonic plague. The odd word “bubo” comes from the Greek boubon meaning groin or swollen groin. To be distinguished from a bubu (or boo-boo) which is a small injury sustained by a child.