It also, the team quickly realized, presented them with a rare opportunity. “The wonderful thing about the cholera victims—not so wonderful for them, of course—is the fact that a number of them were buried encased in liquid lime,” Larsen said. Over the centuries, the lime, once used to mask the smell of the bodies as they decomposed, hardened into a shell than enveloped the skeletons and the soil that surrounded them.

In most cases, bones can provide only limited insight into the diseases that sickened their owners. “The problem with skeletons is that they only display a record of chronic diseases. The person has to be alive long enough for the disease to have an impression on bone,” Larsen explained. “The diseases we can’t look at [in skeletons] are those that kill you right away, like plague and smallpox and cholera.” But the soil around these particular bones, trapped in its lime casing from the beginning, may contain a different kind of information: the genetic material of Vibrio cholerae, the bacterium that causes cholera. If they could find the DNA, the researchers reasoned, they may be able to reconstruct the genome of the 19th-century strain.

“Understanding how pathogens evolve through time gives us a picture of the evolutionary history that we can’t really glean from just modern-day sequence data,” said Hendrick Poinar, a professor of genetics and anthropology at McMaster University and the lead researcher on genetic screening for the project. In addition to scanning soil from the gravesite, Poinar says, he also analyzes the bodies’ teeth for genetic material—altogether, he says, “we look for [the DNA from] over 3,000 different bacterial, viral, and fungal species” in the samples that pass through his lab.

Last year, Poinar and his colleagues published an article in the journal The Lancet Infectious Diseases outlining a similar project, unrelated to Larsen and the Field School, with the remains of bodies from the Aschheim-Bajuwarenring cemetery in Bavaria, Germany. Using DNA taken from the teeth of the victims of the Plague of Justinian, a bubonic-plague epidemic that lasted from the sixth to eighth centuries, the researchers were able to sequence the genome of Y pestis, the bacterium that caused the epidemic, enough to determine that it was a different strain from the outbreaks that would take place centuries later (“the Black Death” in the 1300s-1600s, and a third outbreak in the 1800s and 1900s).

“If you have a site that contains remains and that level of continuity over six, seven, 800 years, then there is this distinct possibility to look at population health and pathogen burden over time,” Poinar said. “That’s really contributing tremendously to knowledge of our evolutionary fight with pathogens.”