Plague germs teased from mediaeval cadavers in a London cemetery have shed light on why the bacterium that unleashed the Black Death was so lethal and spawned later waves of epidemics.

The DNA of Yersinia pestis shows, in evolutionary terms, a highly successful germ to which the population of 14th-century Europe had no immune defences, according to a study published Wednesday in the British journal Nature.

It also lays bare a pathogen that has undergone no major genetic change over six centuries.

"The Black Death was the first plague pandemic in human history," said Johannes Krause, lead researcher and a professor at the University of Tuebingen, Germany.

"Humans were (immunologically) naive and not adapted to this disease," he said in an email exchange.

No bug or virus has wiped out a greater proportion of humankind in a single epidemic than the Black Death.

Brought to Europe from China, it scythed through the continent from 1347 to 1351, killing about 30 million people -- about one in three of Europe's and nearly one in 12 of the world's population at the time.

Remarkably, more recent variants of the bacterium hardly vary compared to the original microbe, says the paper.

"Based on the reconstructed genome, we can say that the mediaeval plague is close to the root of all modern human pathogenic plague strains," said Krause.

"The ancient plague strain does not carry a single position that cannot be found in the same state in modern strains."

This deep similarity between ancient and modern plague calls into question the long-held assumption that virulence-enhancing mutations are what made Y. pestis so deadly to the Middle Ages.

Like Native American Indians who were exposed to smallpox, Europeans had never been exposed to the bacterium, said Krause.

"Plague was among the strongest sources of selection on the human population in the last few thousand years," he added. "People who were less susceptible due to mutations might have survived, and these (beneficial) mutations may have spread."

Another likely factor that worsened the Black Death's toll was social conditions, which were far worse compared to the 18th or 19th centuries. Poverty and malnutrition were rampant, and even the concept of hygiene was non-existent.

The onset of the so-called "Little Ice Age" could also have favoured the spread of the disease which, like many pathogens, travels more quickly in cold climes.

The same goes for the rats that carried the blood-sucking insects -- fleas or lice, perhaps both -- that transmit the disease.

Indeed, the species of rodent, Ratus ratus, that sowed terror across a continent in the 14th century is not the same as the one that transports plague today, Ratus norvegicus, Krause said.

The 1918-19 Spanish flu pandemic killed by some estimates 50 million people. In absolute terms, this was the deadliest pandemic in human history.

But with a world population that was close to two billion, the toll in relative terms was far smaller than that of the Black Death, when the number of humans was in the hundreds of millions.

The first outbreak of plague occurred in China more than 2,600 years ago before reaching Europe via Central Asia's "Silk Road" trade route, according to a molecular "family tree" -- mapped out last year -- of 17 Y. pestis strains.

It then spread to Africa, probably by an expedition led by Chinese seafarer Zhang He in the 15th century.

In the late 19th century, plague came to the United States from China, arriving in the ports of California via Hawaii, according to this evidence.