A plague, also known as an epidemic or a pandemic, was a widespread outbreak of an infectious disease.
In 2164, a plague struck Sauria in the 22nd century. (ENT - Rise of the Federation novel: Tower of Babel)
In 2259, a plague struck Argylus, killing everyone over the age of thirteen. (TOS comic: "Child's Play")
In 2266, Tomasina Laubenthal used one of the Malkus Artifacts to unleash a plague on Alpha Proxima II. (TOS - The Brave and the Bold, Book One novella: The First Artifact)
In 2267, the Nisus colony was struck by a plague. (TOS novel: The IDIC Epidemic)
In 2373, the Federation was struck by the Virogen plague. (TOS novel: Avenger)
The Kevrata were ravaged by the bloodfire plague for many generations. (TNG novel: Death in Winter)