Bubonic plague, known as "the black death" because of the blackened lymph nodes (buboes) of its victims, swept through Europe during the middle ages killing tens of thousands, decimating cities and striking fear and panic in the populace. From the reservoir of cyclic disease in rodents, the infectious agent Yersinia pestis is spread to humans through the bite of the flea. Many cities at that time were densely populated with very poor sanitation and flourishing colonies of rats. Fleas made their way happily between their human and rodent hosts, ultimately starting and perpetuating the great epidemic waves which changed history. Plague exists in two other forms besides the bubonic--septicemic in which the organism spreads through the blood without necessarily having the swollen lymph nodes, and pneumonic plague characterized by a highly fatal pneumonia. In this form the disease can be spread from human to human directly by droplets produced by coughing.