The plague is a highly infectious disease due to a bacterium carried by rodent fleas. It causes greatly enlarged lymph nodes (buboes, hence bubonic plague), sepsis with fever, prostration, and coma; plague pneumonia is particularly severe. If untreated, death is common and epidemics occur in areas of overcrowding and poverty. It still occurs on a small rural scale in the Far East; massive epidemics such as the Black Death, which perhaps halved the population of Europe in the mid-fourteenth century, are rare. Rat and flea control, disinfection and antibiotics are the mainstay of current prevention and treatment.