The plague bacillus became less virulent, prolonging the duration of two major pandemics

Scientists at the Institut Pasteur and McMaster University have discovered that the evolution of a gene in the bacterium that causes bubonic plague, Yersinia pestis, may have prolonged the duration of two major pandemics. They have demonstrated that modifying the copy number of a specific virulence gene increases the length of infection in affected individuals. It is thought that this genetic change may prompt longer periods of contagiousness in less densely populated environments, in which the time of transmission from one individual to another is inevitably longer. This genetic variation has been observed in strains of each of the two major plague pandemics, hundreds of years before they eventually faded out. The study was published in the journal Science 

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