Plague is a disease caused by the bacterium Yersinia pestis. It occurs mainly in rodents, such as rats and squirrels, but it can be transmitted from rodents to humans by the bite of an infected flea. Plague was the cause of some of the most-devastating epidemics in history, including the Black Death of the 14th century.

In the second case “plague” has the same meaning as the word epidemic. Plague is caused by tiny living things called bacteria. It is mostly a disease of rats. Fleas that bite infected rats pick up the plague bacteria. They then spread it to other rats or to people through bites.

definition 1: a deadly disease, sudden invasion of harmful insects, or any terrible thing that harms many people. A plague of locusts is threatening the country's main food crop. similar words: affliction, epidemic. definition 2: a disease that spread quickly and killed many people in former times. related words: blight, curse, disease, poison, worry.