Plague is an infectious disease of animals and humans caused by the bacterium Yersinia pestis. The disease is spread to animals and humans through infected fleas, by direct contact with the tissues or body fluids of a plague-infected animal/human, or by inhaling infectious droplets from persons or animals. There are three forms of the plague: bubonic, pneumonic, and septicemic. They are all caused by the same bacterium, but are transmitted differently and their symptoms differ. Pneumonic plaque is the least common but most rapidly fatal form of the plague.