While the deadliest known epidemic in history was pretty dangerous to everyone around, it turns out the Black Death did not kill indiscriminately, as popular wisdom holds. Instead it targeted people who were weak to begin with.

Until now, many researchers believed everyone was at equal risk for the plague that wiped out an estimated 75 million people between 1347 and 1351.

"The fact that it killed so many people is why people assumed it killed indiscriminately," said Sharon DeWitte, a biological anthropologist at the University at Albany who made the discovery. "But actually it killed selectively. Even with diseases as horribly devastating as the Black Death, even then there are differences in individuals' risk."

Black Death cemetery

DeWitte and James Wood of Pennsylvania State University studied 490 skeletons from the East Smithfield Black Death cemetery in London, which was dug specifically for plague victims. The cemetery was excavated in the 1980s, and the skeletons were cleaned and stored for research. DeWitte and Wood recently went to London to examine the bones for lesions — holes, defects or malformations that indicate general frailty.

For a control, they compared the Black Death skeletons to samples from other cemeteries in which people were buried before the plague. The scientists estimated the victims' ages when they died, and used computer models to calculate the link between bone lesions and risk of death.

In both populations, DeWitte found a correlation. As she expected, the link between frailty and risk of death was stronger in non-plague victims, because in general, unhealthy people are more likely to die than well people. However, she also found a connection between frailty and death in the Black Death skeletons, meaning that people who were unwell before they caught the plague were more likely to die of the disease than healthy people.

The researchers assume infirm people succumbed more often because poor nutrition compromised their immune systems' ability to fight the disease.

"It makes sense that the Black Death would kill people who are already weak," DeWitte told LiveScience. "What's important is that we've provided quantitative evidence to counter the assumption that it killed indiscriminately."

The findings were published in the Jan. 28 issue of the journal, Proceedings of the National Academy of Sciences.

A perfect sample

When the Black Death swept through Europe it wiped out nearly one-third of the European population, according to published reports.

DeWitte said the new finding is particularly important because it eliminates the possibility that victims of the plague represent an unbiased sample of the fourteenth-century population, which researchers wanted to use to study general population trends.

"Some hoped that the cemeteries represented a perfect sample," DeWitte said. "That's out the window. It demonstrates that we should never expect any cemetery to provide an unbiased sample of past populations," In the Black Death cemetery, as in most cemeteries, those in poor health are over-represented.

DeWitte said the finding could help scientists prepare for future epidemics.

"This is relevant because there are a lot of diseases that are emerging and becoming much more prevalent," she said. "The fear is that these will be massive killers and everyone will be at equal risk. Being aware that there will be some people at higher risk means we can take measures to reduce their risk."