These tougher strains are raising “serious concerns,” according to the study, published in the March issue of the journal Infection, Genetics, and Evolution.

The findings were announced on March 15 as archaeologists unearthed a “Black Death” grave in London, containing more than a dozen skeletons of people suspected to have died from the plague. The victims are thought to have died during the 14th century and archaeologists anticipate finding many more as they excavate the site.

The plague is a highly contagious disease affecting the lungs. Population levels suffered globally due to the plague, with around 75 million people globally perishing during the 14th century Black Death, according to researchers. The plague has returned episodically in recent decades, although, thanks for modern medical care, fatalities have not surpassed a few dozen in any recent outbreak.