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.