In general, names of diseases are not capitalized unless they contain a proper name, such as Crohn's disease. So, it's the plague. According to Merriam-Webster, it's the plague, the bubonic plague, or black death.

Is bubonic plague capitalized?

Specific periods, eras, and historical events that have proper names should be capitalized. Most of the World War I veterans are now deceased. In the Middle Ages, poor hygiene was partly responsible for the spreading of bubonic plague.

Is Black Plague a proper noun?

proper noun

The great epidemic of bubonic plague that killed a large part of the population of Europe in the mid 14th century.