Primarily a disease of rodents, plague in human beings originates either by contact with infected rodents, most commonly rats, but more likely their fleas. This is one reason why plague follows ecosystem disruption or physical devastation, because otherwise humans rarely intrude into areas where plague is being transmitted and plague-infected rodents rarely intrude on areas of human settlement.
The disease in man has three clinical forms: bubonic, characterized by swelling of the lymph nodes; pneumonic, in which the lungs are extensively involved; and septicaemic, in which the blood stream is so strongly invaded by the bacterium that death ensues before the bubonic or pneumonic forms have had time to appear. With prompt antibiotic treatment, plague can almost always be cured; but mortality for untreated cases can exceed 50 - 60%.
When an infected flea bites a human it passes the plague bacterium Yersinia pestis (formerly Pasteurella pestis) into the site of the bite. Sporadic cases and epidemics are reported in countries with known natural foci of the disease. These provide plague with ample opportunities to re-emerge, and the appearance of resistance phenomena gives rise to the fear that present methods of combating plague may eventually lose their effectiveness. Although the third plague pandemic, which began at the end of the nineteenth century and, aided by steamships, invaded the entire world, has now come to an end and although continuous world-wide surveillance of human and rodent plague is imperative if great human and economic distress is to be avoided, modern communications bring a growing danger that outbreaks of plague will occur in areas hitherto free from the disease. The new technique of freighting in containers presents a definite threat. Under this system, cargo packed in infected areas cannot be inspected or treated en route, and there may be no facilities for inspection and treatment at its destination. If plague-infected rodents and fleas are present in the containers, they may well survive and cause human or rodent plague. The transport of such containers by air may present a particular hazard. With the growing use of high-speed passenger aircraft, moreover, there is an ever increasing possibility that travellers in the incubation phase of plague will disembark in a hitherto plague-free area.
In the 14th century, the Black Death killed about one-third of the population of Europe. The last major outbreak was in China early in the 20th Century.
Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year. The reported frequency has grown; for example in 1982, 713 cases (including 36 deaths) were reported to the World Health Organization. Of these, 250 cases and 31 deaths were recorded in Africa (Uganda, Madagascar, Tanzania, Zimbabwe). After being dormant for 10 years, human plague was noted in South Africa. In 2017, over 100 people died of plague in Madagascar. In Asia, human plague has been notified from Burma and Vietnam; however, local outbreaks could appear in any country of Southeast Asia, especially in those areas which are enzootic for wild rodent plague and where disinsectization of populated localities is not carried out regularly enough. Human plague has also been noted in four countries in the Americas (Brazil, Bolivia, Peru, USA).
More worrying for areas currently free of the plague was one diagnosed case in Kazakhstan in 1993 in a woman after she cut up a hare, and unconfirmed reports of other cases. The disease is endemic in the animal population of small areas of Siberia, Mongolia and central Asia, but rarely spreads to humans.
In the United States, the last urban plague epidemic occurred in Los Angeles in 1924-1925. Since then, human plague in the United States has occurred as mostly scattered cases in rural areas (an average of 10 to 15 persons each year). This is one of the diseases considered of value in biological warfare. More than 10 institutes and thousands of scientists were reported to have worked with the plague in the former Soviet Union.
All that separates the virulent and non-virulent forms of the disease is a mutation in the bacteria. The bacteria mutates to its virulent form, spreads through high density populations along trade routes, migration routes and in times of war. It then subsides and mutates back to its non-virulent form to await the next pestilent opportunity.