It’s hard to imagine an infectious disease so horrible that it kills every second person. And not every second person it infects: every second person, period. That was the “Justinian Plague” of sixth-century Europe. Another wave of bubonic plague, beginning in 1347, killed a third of Europeans. Imagine the terror that invoked, since the cause was completely mysterious.
Today’s New York Times reports on two recent studies of black death. In one, by Stephanie Haensch et al., researchers sequenced plague DNA from medieval “plague pits” where bodies were thrown. They not only identified the causal organism as the bacterium Yersinia pestis (definitively settling a long-standing debate), but also found that it invaded Europe at least twice: once from the north and once from the south.
And in the new Nature Genetics, Morelli et al. do a phylogenetic reconstruction of the plague’s genetic history from geographically widespread sequences. The pathogen apparently originated in or near China and, using a molecular clock, the researchers traced and dated the successive waves of invasion that caused epidemics of Black Death.
Here’s the final paragraph of Nicholas Wade’s report; the “slaughters by accident” is apt but not completely accurate: while most plague is transmitted by fleas that bite infected rodents and then humans, the pneumonic form (which occurs when the disease infects the lungs) can go directly from person to person.
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.
Fig. 1. The terrifying costume of a medieval plague doctor. Long coat, boots, glass goggles, and a beak stuffed with herbs and spices to mask the stench.
Haensch S, Bianucci R, Signoli M, Rajerison M, Schultz M, et al. 2010. Distinct clones of Yersinia pestis Caused the Black Death. PLoS Pathog 6(10): e1001134. doi:10.1371/journal.ppat.1001134
Morelli, G. et al. 2010. Yersinia pestis genome sequencing identifies patterns of global phylogenetic diversity. Nature Genetics doi:10.1038/ng.705