"In some cases death came immediately; in others, after many days," .
"With some, the body broke out with black pustules about as large as a lentil and these did not survive even one day, but all succumbed immediately. Vomiting of blood ensued in many, without visible cause, and immediately brought death."
What Procopius observed first hand was the Plague of Justinian, named after the Eastern Roman emperor he served and who contracted the disease but survived.
The first of three plague pandemics to have ravaged humanity, it killed between 25 and 100 million people across Asia, North Africa and Europe.
After an initial two-year rampage, it returned in waves before mysteriously disappearing in the middle of the eighth century.
Using DNA teased from 1,500-year-old teeth of plague victims buried in Germany, scientists have reconstructed the genetic profile of the killer and say its ability to mutate is a warning for people today.
Different from the Black Death
The strain of Yersinia pestis germ which caused the Plague of Justinian was different from the strain that triggered the Black Death in the 14th century, killing an estimated 30 million Europeans, they found.
It is also distinct from the Y. pestis strain that caused a third outbreak of plague in the late 19th century, and which was likely to have been a genetic offshoot from the Black Death microbe.
The evidence confirms, as expected, the role of rats as the germ's "reservoir" or natural source, according to a paper published in the journal The Lancet Infectious Diseases.
It also throws up new avenues for exploring the dynamics of plague: why a pandemic erupts and dies out, and how a novel strain emerges, becoming a threat against which the immune system has no defence.
?We know the bacterium Y. pestis has jumped from rodents into humans throughout history, and rodent reservoirs of plague still exist today in many parts of the world," said Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University.
"If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggests it could happen again."
Wagner added: "Fortunately we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large scale human pandemic."
'Intrinsically high virulence'
The samples used in the research came from two individuals buried in a mediaeval graveyard in Aschheim, Bavaria, and whose remains were dated to around 504 and 533 AD respectively.
Assembling their genomes is a feat, for they are the DNA codes of the oldest pathogens obtained to date.
Comparing the mediaeval strain to 131 Y. pestis bacteria in a database of the Black Death and 19th century plagues, the investigators concluded the Plague of Justinian came to the West out of Asia, as did the two later pandemics.
The 6th-century germ came equipped with a tool kit "of intrinsically high virulence," they write.
Helped by trade, poor hygiene and lack of immune defence among the host populations, this made the bacterium extraordinarily lethal, killing up to 40 percent of those it infected.
Less clear, though, is whether the pandemic germ was a cataclysm that almost literally came out of nowhere, or the culmination of a buildup -- possibly over centuries -- from isolated outbreaks.
As to why the plague disappeared and re-emerged seven centuries later, two theories are held.
One is that after infecting so many people, levels of immunity built up in the population, making it difficult for the germ to spread successfully.
Another is natural variation in climate which disrupted the conditions in which the germ spread.
Before each of the three plague pandemics, there was a spell of exceptional rainfall, and the climate was also unstable when these outbreaks ended, the study says.
Plague remains a background problem in some countries. Between 1,000 and 2,000 cases of plague are reported to world health watchdogs each year, although this figure is probably conservative, according to the US Centers for Disease Control and Prevention (CDC).