Plague is a disease caused by the bacterium Yersinia pestis, which is thought to be the cause of three world pandemics in history: the Plague of Justinian in the 6th century A.D., which broke apart the Roman Empire for good; the Black Death in the 14th century, which wiped out between a quarter and a half of Europe's population; and the Third Pandemic in the late 19th and early 20th centuries, during which the responsible bacterium was isolated.
The term "plague" was also used historically to refer to any epidemic of disease. These are caused by germs; a fact that has been known since the late 19th century and the validation of the germ theory of disease, which, like evolution, is not a hypothesis but rather demonstrated fact on the order of the theory that rain is caused by clouds.
 Religion and plagues
Some Christian fundamentalists persist in suggesting that when others experience outbreaks of disease, that this is a punishment from God, much like the plagues visited upon Egypt; Jerry Falwell said this of the AIDS epidemic among gays, roughly following Fred Phelps's line.
They do this despite the fact that their congregations are filled primarily with elderly people who suffer an epidemic of diseases of excess such as artheriosclerosis. Hypocrisy is, ironically, one of the vices most detested by Jesus according to biblical accounts (Matthew 23 is one famous example). Even more ironically, Jesus also spoke against people who ascribed natural disasters to God punishing their victims (Luke 13:1-5).
 Artificial plagues
Biological warfare is an unfortunate fact of modern life. According to the Westboro Baptist Church, deaths experienced by US combat soldiers at the hands of the Taliban are God's punishment to America for abandoning "Godly ways". Thus, a hypothetical Taliban biological terrorist attack would be, according to such fundamentalists, a plague from God, and Al Quaeda would be the vicars of the Lord.