The plague of 1665, mentioned by Greg Stephens and others, reminded me that Samuel Pepys, who lived through this time, best known for his diaries, was a ballad collector. His collection of over 1800 was left to Magdalene College (See Firth, Introduction to "An American Garland").

The thorough explanation by Philip Hiscock should be saved because the topic will undoubtedly come up again. I can't bring up any old threads today, but titles suggest that the subject has come up several times before.
I am intrigued by a song title in the DT, "Ring-Around-The-Rosy Rag," and look forward to reading the lyrics when the DT-Forum is functioning again.