This is a written song about the Bubonic Plague vs. the Black Death, that I heard at a session and tracked down here:

http://seananmcguire.com/songbook.php?chords=117

I contacted the songwriter to see if there were any references for the epidemiological claims, and was referred to the book
The Return of the Black Death: The World's Greatest Serial Killer, by Christopher John Duncan and Susan Scott.