A plague, also known as an epidemic or a pandemic, was a widespread outbreak of an infectious disease.
In 1666, a plague was artificially implanted in England by a trio of rogue Terileptils, who implanted genetically modified bacteria into black rats and used their fleas to spread the contagion. However, the Fifth Doctor inadvertently (and presumably fatally) immolated the Terileptils in the Great Fire of London, which he then used to destroy the remaining vials containing the plague. (TV: The Visitation)
In the 1970s, the Silurians under Wenley Moor tried to create an epidemic with their Silurian virus that would wipe out humanity, but the Third Doctor discovered a cure. (TV: Doctor Who and the Silurians)
In the early 21st century, diseases became more numerous and more virulent due to high pollution levels. These new diseases were known collectively as "the plague". Ruby Duvall's mother died from one of the plague diseases. (PROSE: Iceberg)
In 2157, the Daleks used meteorites to spread a plague on Earth prior to their invasion six months later. Whole continents of people died, including Asia, Africa and South America. (TV: The Dalek Invasion of Earth)
After the Eternal War ended, Gallifrey was ravaged by a plague which only a few survived. It killed everyone who knew that it was Rassilon's mistake which had brought the Yssgaroth into the universe. (PROSE: The Pit)
In the 13th century the immortal human Ashildr lost her three children Essie, Johann, and Rue to the bubonic plague. She recorded these events in her journals as a reminder to herself to never have any more children. (TV: The Woman Who Lived, PROSE: The Triple Knife)