Pandemics of the plague have broken out three times in recorded history. A pandemic is a large-scale epidemic. In an epidemic, a disease is confined to certain locations, such as cities or regions. In a pandemic, people are afflicted with a disease over entire countries or continents. Given time, a pandemic can circle the globe. After an initial outbreak that lasts several years, the disease virtually disappears, only to break out in periodic epidemics in the following years. This cycle can be repeated for decades or even centuries before the disease disappears completely.

The first pandemic of plague began in A.D. 542 during the reign of Justinian, Emperor of the Byzantine Empire; for this reason it is called Justinian's plague. It seems to have begun in Egypt and spread northward through the eastern Mediterranean region. In the following years it swept through Europe, central...