Yersinia pestis affecting mammals is called plague, and is renowned for causing three pandemics that killed about 200 million people in past centuries. This disease, and its hosts and vectors, have had profound effects on human history. The first pandemic, known as the Justinian Plague , affected the Byzantine Empire, specifi cally Egypt, the Middle East, and Mediterranean Europe. It lasted from about 540 to 700 A.D. and caused about a 50% – 60% reduction in human populations in the affected area. The second pandemic, called the Black Death , occurred in the 1300s and killed about 75 million people, or about one - third of the people in Europe. The third (unnamed) pandemic started about 1855 in China, but was soon spread to seaports in many countries around the world. India was particularly affected, realizing about 12.5 million deaths in 20 years. It was during this outbreak that the bacterial cause of the disease was discovered. It was not until 1908 that the involvement of rats was established, though it was suspected for centuries because massive deaths of rats typically preceded the occurrence of disease in humans. Technically, the third pandemic has not been terminated as there are continuing cases, particularly in Vietnam, where up to 250,000 cases are estimated to have occurred in the second half of the 1900s. Thus, plague remains an occasional problem, especially in less developed countries. Plague outbreaks typically subside when most susceptible hosts perish, or surviving hosts develop immunity.
Among humans, this bacterium causes three forms of plague, based on the nature of the infection. Bubonic Plague is transmitted by fleas to the skin, swellings called buboes occur, and lymph nodes are commonly affected. Septicemic Plague occurs when the flea injects the bacilli directly into a capillary vein, and the infection becomes general rather than localized in the lymph system. Pneumonic Plague , is caused by transmission of the bacilli through coughing or sputum following involvement of the lungs.
Plague normally resides in wildlife, specifically rodents, and is transmitted by fl eas. Virtually all mammals can become infected with the pathogen, though susceptibility varies widely, and nearly all non - rodent and non - lagomorph species are considered to be incidental hosts. Maintenance of the plague at relatively low levels in wild rodents is sometimes referred to as Sylvatic Plague or Campestral Plague . In the USA, it persists at low levels in arid western rural environments but occasionally erupts to attain higher levels, even affecting humans in urban environments as far east as New York City. In the enzootic cycle the reservoir consists of 30 – 40 species of rodents, with birds, rabbits, carnivores, and primates unaffected. In urban areas, outbreaks usually involve urban rats. Outbreaks based on urban rat hosts is referred to as Murine Plague . Around the world, more than 200 species of rodents can be affected.
