The collective noun plague (a word for a dangerous disease that spreads quickly) is used to group things that are harmful and have become too common (a plague of violence, a plague of accidents); or things that have become too great in number, seemingly our to control (a plague of locusts, a plague of rats).

a plague of frogs
the great plague of eyam

The black plague. The plague was transferred by rats.

Because they tried to cure the plague. Doctor who cures the plague --> plague doctor.

the-plague of gnats