Many historians have assumed that Europe’s deadliest plague, the Black Death of 1347 to 1351, killed indiscriminately, young and old, hardy and frail, healthy and sick alike. But two anthropologists were not so sure. They decided to take a closer look at the skeletons of people buried more than 650 years ago.

Their findings, published on Monday in The Proceedings of the National Academy of Sciences, suggest that the plague selectively took the already ill, while many of the otherwise healthy survived the infection.

Although it may not be surprising that healthy people would be more likely to survive an illness, it is not always the case. The Spanish flu of 1918 killed thousands of healthy people in their prime while sparing many children and the elderly, whose weaker immune systems did not overreact to the infection. Sexually transmitted infections like H.I.V. disproportionately affect the strongest and healthiest, for the obvious reason that they are the most sexually active.