The second plague pandemic was a major series of epidemics of plague that started with the Black Death, which reached Europe in 1348 and killed up to a half of the population of Eurasia in the next four years. Although the plague died out in most places, it became endemic and recurred regularly. A series of major epidemics occurred in the late 17th century, and the disease recurred in some places until the 19th century. After this, a new strain of the bacterium appeared as the third pandemic.
Plague is caused by the bacterium Yersinia pestis, which exists in parasitic fleas of several species in the wild and of rats in human society. In an outbreak, it may kill all of its immediate hosts and thus die out, but it can remain active in other hosts that it does not kill, thereby causing a new outbreak years or decades later. The bacterium has several means of transmission and infection, including through rats carried on ships or vehicles, fleas hidden in grain and—in its more virulent forms—transmitted by blood and sputum directly between humans.