Bubonic plague. Swollen lymph node, or bubo, under the arm of a patient with bubonic plague. Plague is caused by the bacteria Yersinia pestis, which is spread to humans by bites from infected fleas carried on rats. Infection is rapid, causing swollen lymph nodes and leading to septicaemia, which is always fatal, and pulmonary infection, which can be fatal within a day. Prompt treatment with antibiotics can effect a cure.