Forms of the plague still circulate among humans, though none are as deadly as the strain from the 14th century that evolved from a harmless non-pathogenic bacterium that lives in soil, Yersinia pseudotuberculosis. Researchers have begun to track changes in the pathogen’s evolution and virulence, mapping out a phylogenetic tree, though changes in the bacterium’s genome is perhaps not the most significant. Humans have adapted to the disease both genetically and in practice, building up immunity and alleviating cultural conditions like malnutrition.
The Y. pestis genome consists of a single 4.6 megabase-long chromosome, and three small plasmids. Only 97 of these bases have changed in the 660 years since the Black Death, and only a dozen in genes. This supports the notion that the deadliness of the Black Death was primarily a result of medieval conditions. The sequencing of the plague is both a significant advance in infectious biology and scientific technology, lead by researchers at McMaster University and the University of Tubingen in Germany.