The remains of 36 bubonic plague victims from a 16th century mass grave in Germany provide evidence that evolutionary adaptive processes, driven by the disease, may have conferred immunity on later generations of people from the region.

The researchers collected DNA samples from the inner ear bones of individuals in a mass grave in the southern German city of Ellwangen which experienced bubonic plague outbreaks in the 16th and 17th centuries. Then they took DNA samples from 50 current residents of the town. They compared their frequency spectra - the distribution of gene variants in a given sample - for a large panel of immunity-related genes and found that innate immune markers increased in frequency in modern people from the town compared to plague victims.

Among the current inhabitants, the team found evidence that a pathogen, likely Yersinia pestis which causes bubonic plague, prompted changes in the allele -  a variant form of a gene -distribution for two innate pattern-recognition receptors and four Human Leukocyte Antigen molecules, which help initiate and direct immune response to infection. 

The findings are the first evidence that evolutionary processes, prompted by Y. pestis, may have been shaping certain human immunity-relevant genes in Ellwangen and possibly throughout Europe for generations. And since the plague tormented Europe for nearly 5,000 years, the study suggests that these immunity genes may have been pre-selected in the population long ago but recently became selected through epidemic events.

A silver lining in COVID-19 is hard to find, but it didn't kill off 25 percent of the population. And while coronavirus pandemics are common, SARS-CoV-2 was the third in the last 17 years and the virus was only recognized as distinct from the common cold in the 1960s, there will be spikes in severity in the future. So biological herd immunity among those who are less impacted will be a positive.