The term plague, in human history, refers to an epidemic disease causing a high rate of mortality, i.e. a pestilence. An epidemic — disease outbreaks that strike a large number of people in an area at the same time, may also become a pandemic when it spreads over a wide geographical area or throughout many countries. Bubonic plague, typhus, smallpox, cholera, yellow fever, influenza, scarlet fever, malaria, diphtheria, and poliomyelitis are some infectious diseases that have resulted in epidemic or pandemic outbreaks.

Plagues of disease are a major factor in the development of human civilization, impacting and altering the course of wars, migrations, population growth, urbanization, industry and cultural development. The term carries such extreme connotations that it is often synonymous with a “calamity”, projecting an image of a disastrous evil or affliction.

Plague retains an important place in human history. Humanity has always been vulnerable to and fearful of infectious disease, which has wrought misery, devastation, and havoc throughout the world since ancient times. Times of pestilence have interrupted human affairs and brought great suffering which, in historic times, has often been described and reported in detail. Outbreaks result in extreme loss of life and damage to institutions and economies.

In early cities, large populations were concentrated into crowded communities that often had limited access to fresh water and unregulated disposal of waste. In these communities, waves of disease, whatever the agent of infection, created terror and panic. Accounts of armies that were depleted or defeated by bouts of infection stretch back to the ancient world, and epidemics have frequently ruined the plans and ambitions of military leaders.

Two well known examples of the impact of disease on history are the Black Death, which periodically visited various peoples throughout Asia and Europe between the fourteenth and seventeenth centuries, and the overwhelming pandemics of measles and smallpox, as well as other Eurasian diseases, which Europeans brought to peoples in the New World. Both of these devastating occurrences were made more severe by the fact that each population was “biologically naïve”. When a population that has been relatively isolated is exposed to a new disease or a group of new diseases, it has no inborn resistance; the body of people succumbs at a much higher rate, resulting in what is known as a “virgin soil” epidemic.

During the disease outbreak of the Middle Ages, the single word “plague” was associated with a disease which reached epidemic and even pandemic proportions in Asia and Europe. The disease has most commonly been identified as bubonic plague and its variants, the virulent contagious febrile disease caused by the bacillus Yersinia pestis, which is sometimes spread by fleas from rodents to humans. However, recent investigations have cast doubt on this theory, with an alternate hypothesis suggesting that ongoing outbreaks were caused by a viral hemorrhagic disease, perhaps similar to Ebola.

The disease was known in isolated pockets in Asia but had rarely been seen west of the Byzantine empire. Sweeping outbreaks in Medieval Europe drastically decreased the population, disrupting several vital civilizations and are considered to have significantly altered the course of human affairs.

Before the European arrival, the Americas had been isolated from the Eurasian-African landmass. First contact between Europeans and native people of the American continents brought overwhelming pandemics of measles and smallpox as well as other Eurasian diseases. These diseases spread rapidly among native peoples, often ahead of actual contact with Europeans, and led to a drastic drop in population and the collapse of American cultures. Smallpox and other diseases invaded and crippled the Aztec and Inca civilizations in Central and South America in the sixteenth century. This disease, with loss of population and death of military and social leaders, contributed to the downfall of both American empires and the subjugation of American peoples to Europeans.

The danger posed by epidemic disease has not been eliminated by modern health and hygiene practices. The ever-enlarging human population, rapid international transportation, developing resistance to medication by known disease agents, insect resistance to insecticides and medical complacency have both generated new strains of old diseases and increased the possibility of epidemics caused by emerging new diseases.