What caues the plague?

Answer 1

"The plague" doesn't mean too much as many diseases and even animals - like locusts - have been said to cause plagues. But -

There is debate as to what the famous "black death" was as the normal form of Bubonic Plague, which is what it has traditionally been associated with, does not normally behave as the disease did during that time.

Yersinia pestis is the bacillus that causes plague.