New York: The great waves of plague that twice devastated Europe and changed the course of history had their origins in China, a team of medical geneticists reported Sunday, as did a third plague outbreak that struck less harmfully in the 19th century.
And in separate research, a team of biologists reported conclusively this month that the causative agent of the most deadly plague, the Black Death, was the bacterium known as Yersinia pestis. This agent had always been the favored cause, but a vigorous minority of biologists and historians have argued the Black Death differed from modern cases of plague studied in India, and therefore must have had a different cause.
The Black Death began in Europe in 1347 and carried off an estimated 30 percent or more of the population of Europe. For centuries the epidemic continued to strike every 10 years or so, its last major outbreak being the Great Plague of London from 1665 to 1666. The disease is spread by rats and transmitted to people by fleas or, in some cases, directly by breathing.
One team of biologists, led by Barbara Bramanti of the Institut Pasteur in Paris and Stephanie Haensch of Johannes Gutenberg University in Germany, analyzed ancient DNA and proteins from plague pits, the mass burial grounds across Europe in which the dead were interred. Writing in the journal PLoS Pathogens this month, they say their findings put beyond doubt that the Black Death was brought about by Yersinia pestis.
Dr. Bramanti's team was able to distinguish two strains of the Black Death plague bacterium, which differ both from each other and from the three principal strains in the world today. They infer that medieval Europe must have been invaded by two different sources of Yersinia pestis. One strain reached the port of Marseilles on France's southern coast in 1347, spread rapidly across France and by 1349 had reached Hereford, a busy English market town and pilgrimage center near the Welsh border.
The strain of bacterium analyzed from the bones and teeth of a Hereford plague pit dug in 1349 is identical to that from a plague pit of 1348 in southern France, suggesting a direct route of travel. But a plague pit in the Dutch town of Bergen op Zoom has bacteria of a different strain, which the researchers infer arrived from Norway.
The Black Death is the middle of three great waves of plague that have hit in historical times. The first appeared in the 6th century during the reign of the Byzantine emperor Justinian, reaching his capital, Constantinople, on grain ships from Egypt. The Justinian plague, as historians call it, is thought to have killed perhaps half the population of Europe and to have eased the Arab takeover of Byzantine provinces in the Near East and Africa.
The third great wave of plague began in China's Yunnan province in 1894, emerged in Hong Kong and then spread via shipping routes throughout the world. It reached the United States through a plague ship from Hong Kong that docked at Hawaii, where plague broke out in December 1899, and then San Francisco, whose plague epidemic began in March 1900.
The three plague waves have now been tied together in common family tree by a team of medical geneticists led by Mark Achtman of University College Cork in Ireland. By looking at genetic variations in living strains of Yersinia pestis, Dr. Achtman's team has reconstructed a family tree of the bacterium. By counting the number of genetic changes, which clock up at a generally steady rate, they have dated the branch points of the tree, which enables the major branches to be correlated with historical events.
In the issue of Nature Genetics published online Sunday, they conclude that all three of the great waves of plague originated from China, where the root of their tree is situated. Plague would have reached Europe across the Silk Road, they say. An epidemic of plague that reached East Africa was probably spread by the voyages of the Chinese admiral Zheng He who led a fleet of 300 ships to Africa in 1409.
"What's exciting is that we are able to reconstruct the historical routes of bacterial disease over centuries," Dr. Achtman said.
Lester K. Little, an expert on the Justinian plague at Smith College, said in an interview from Bergamo, Italy, that the epidemic was first reported by the Byzantine historian Procopius in 541 A.D. from the ancient port of Pelusium, near Suez in Egypt. Historians had assumed it arrived there from the Red Sea or Africa, but the Chinese origin now suggested by the geneticists is possible, Dr. Little said.
The geneticists' work is "immensely impressive," Dr. Little said, and adds a third leg to the studies of plague by historians and by archaeologists.
The likely origin of the plague in China has nothing to do with its people or crowded cities, Dr. Achtman said. The bacterium has no interest in people, whom it slaughters by accident. Its natural hosts are various species of rodent such as marmots and voles, which are found throughout China.