Periodically, devastating outbreaks of infectious disease occur, affecting many people in a region at the same time. Such outbreaks are called epidemics. Those of widespread proportions, such as the current AIDS epidemic, are often referred to as pandemics.
People have always been fearful of epidemics and their effects. In China in the 13th century bc, the ruler of Anyang asked his diviners, "Will this year have pestilence and will it be deaths?" In Egypt around 2000 bc, a writer compared fear of the Pharaoh with fear of epidemics. The Old Testament of the Bible refers to several epidemics, including one that affected the Philistines, purportedly as punishment for seizing the Ark of the Covenant. The British Isles were hit by at least 49 epidemics between ad 526 and 1087.
Epidemics can reshape societies, affect the course of military events, and change the balance of power among different groups of people. An epidemic in Athens in 430 bc created chaos in the city and contributed to defeat in its war with Sparta. Among the best known of all epidemics was the Black Death, an epidemic of bubonic plague that broke out in Europe in ad 1347. By 1351 an estimated 25 to 50 percent of the people in Europe had died from the disease. The Black Death depopulated once-flourishing cities, left villages vacant, and caused a decline in cultivated land.
When Europeans began to explore the Americas in the 15th century, they carried along pathogens unknown in the new lands. Smallpox and measles raced through native populations with devastating results. For example, by 1568, only 50 years after Hernán Cortés first reached Mexico, the population of central Mexico had fallen from about 17 million to about 3 million. It is doubtful that Cortés could have conquered the Aztecs as easily as he did had this disaster not befallen the Aztecs.