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The Plague, 1898, by Swiss symbolist painter Arnold BöcklinWikimedia CommonsRats and the fleas they carried may not have been responsible for the rapid spread of the Yersinia pestis bacterium through Europe, the Middle East, and North Africa during the second plague pandemic, the longest in history, which lasted from the fourteenth to the nineteenth century and killed about a third of the population in the regions. Outbreaks still occur today in Asia, Africa, and the Americas, where the disease is transmitted primarily via fleas that have bitten infected rodents or through the air when people who are already infected cough and sneeze. But little is known about the spread of the disease in the Middle Ages. Some studies suggest that fleas (Pulex irritans) and lice (Pediculus humanus) carried by humans may have helped spread the plague during the second pandemic. One supporting factor of this hypothesis is that there are no historical records of increased rodent deaths prior to the cases in humans, as has been observed since the nineteenth century, in the third pandemic. Other studies suggest that the climate in northern Europe in the Middle Ages would have kept rat populations down, as well as recent indications that human parasites can transmit the bacteria. Based on these assumptions, researchers from the University of Oslo, Norway, and the University of Ferrara, Italy, developed three mathematical models to explain the dynamics how the plague spread—by human parasites, by direct transmission, or by rat parasites—in nine epidemics that affected Europe between 1348 and 1813. They then compared the results with historical death records. Transmission via lice and fleas carried by humans best explained the mortality pattern in seven of the nine outbreaks (PNAS, January 16).
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