Well, the good news first: all current known strains of the Bubonic Plague causing bacterium are genetically similar to traces of the Yersinia pestis bacterium found the skeletons of Londoners who died in the 1340s – when the plague killed a third or so of the population of Europe.
That means (we hope!) there is no realistic prospect of a mass plague breakout based on existing bacterial strains – though it should also be noted that the evidence in favour of the effectiveness of current plague vaccines is limited.
But now the bad news: in the sixth century the Justinian Plague contributed to the collapse of the Western Roman Empire by killing off about one-third of the population. From the perspective of historical distance this might be seen as a good thing – the collapse of the cruel imperium – just as the historical legacy of the 1340s Black Death – the breakdown of serfdom and an acceleration of the humanist revolution might seem boons today. But we can be sure that nobody thought so at the time! And in today’s complex and interconnected societies even much smaller death rates would surely tip us over the edge. And the Y. pestis that caused the Justinian Plague has now been shown to have a different genetic makeup from the Black Death’s.
We conclude that the Y pestis lineages that caused the Plague of Justinian and the Black Death 800 years later were independent emergences from rodents into human beings. These results show that rodent species worldwide represent important reservoirs for the repeated emergence of diverse lineages of Y pestis into human populations.
This is, indeed, bad news – as it means that we could be faced with a new plague breakout and have to start the war with the bacterium from scratch (think of Contagion).
The Plague of Justinian was not just one outbreak either – it kept coming back (much in the way that the plague of Black Death did not end in the 1340s – as The Diary Of Samuel Pepys demonstrates.
And if you want an extra worry about climate change here are the final two paragraphs of the paper (the ‘second’ plague is the Black Death, the ‘third’ is 19th and 20th century plague outbreaks):
Why the Y pestis lineage associated with the Plague of Justinian eventually died out is unclear. That it probably caused human epidemics for several centuries suggests
that it was well adapted to human transmission. As a
consequence, several viable explanations for its extinction
might be a scarcity of susceptible hosts (people, or rodents,
or both), insuffi cient numbers of susceptible hosts in a
background of widespread population immunity, or
mutations that arose and spread in the human genome
that conferred resistance to this particular plague strain.
The success of the lineage (or lineages) associated with the
two most recent pandemics is probably attributable partly
to human mobilisation, since increasing trade between
countries and continents is known to have moved people
and rodents with Y pestis infection around the world.
The Plague of Justinian and, indeed, the emergence of
all three plague pandemics, might be tightly linked to
climatic instability; all were preceded by periods of
exceptional rainfall and ended during periods of
climatic stability (around 700–1000 AD in the case of the
Plague of Justinian). Irrespective of the eff ect of climate,
the epidemiological pattern that we propose suggests
that several Y pestis lineages, which are currently
ecologically established in rodent foci worldwide, remain
capable of emerging and igniting epidemics of plague in
human beings, as they have repeatedly in the past.
(Here’s a summary in the New Scientist if you cannot access the Lancet.)