Apparently it's been around for longer than previously thought: http://www.bbc.co.uk/news/health-34603116
And likely still longer, but presumably plagues need a certain density level in a relatively fixed human population to be effective and recognisable as such. Interesting there's a suggested tie-in with the mid-Holocene European "population collapse" circa 8000-4000 cal. BP on that BBC News page, linking to this 2013 paper from Nature (http://www.nature.com/ncomms/2013/131001/ncomms3486/full/ncomms3486.html). The Nature piece doesn't link to disease as such (they were looking at a potential climate cause, which they failed to find), but does draw attention to the events being detectable after human population densities had increased because of the widespread rise in settled agriculture.