WASHINGTON (Reuters) - The Black Death that decimated populations in Europe and elsewhere during the middle of the 14th century may not have been a blindly indiscriminate killer, as some experts have believed.

This undated handout photograph shows the skull of a woman killed by the Black Death in the middle of the 14th century in London. A new analysis of 490 skeletons from a London cemetery for Black Death victims demonstrated that the plague did not place everyone at equal risk, two U.S. scientists wrote in the journal Proceedings of the National Academy of Sciences. While many perfectly healthy people were cut down, those already in poor health prior to the arrival of the plague were more likely to have perished, they found. REUTERS/Jelena Bekvalac and Tania Kausmally/Museum of London/Handout

An analysis of 490 skeletons from a London for Black Death victims demonstrated that the infection did not affect everyone equally, two U.S. scientists said on Monday.

While many perfectly healthy people certainly were cut down, those already in poor health prior to the arrival of the plague were more likely to have perished, they found.

“A lot of people have assumed that the Black Death killed indiscriminately, just because it had such massive mortality,” anthropologist Sharon DeWitte of the University at Albany in New York, said in a telephone interview.

People already in poor health often are more vulnerable in epidemics. “But there’s been a tradition of thinking that the Black Death was this unique case where no one was safe and if you were exposed to the disease that was it. You had three to five days, and then you were dead,” DeWitte said.

The plague epidemic of 1347 to 1351 was one of the deadliest recorded in human history, killing about 75 million people, according to some estimates, including more than a third of Europe’s population.

DeWitte analyzed skeletons unearthed from the East Smithfield cemetery in London, dug especially for plague victims and excavated in the 1980s, for bone and teeth abnormalities that would show that people had health problems before they died of plague.

She found such abnormalities in many skeletons, suggesting these people had experienced malnutrition, iron deficiencies and infections well before succumbing to the Black Death.

The proportion of people with such signs of frailty in the cemetery, compared to those who appeared to have been of robust health before the epidemic, indicated that the infection was somewhat selective in who it killed, DeWitte and Pennsylvania State University anthropologist James Wood reported in the Proceedings of the National Academy of Sciences.

Some experts have thought the Black Death -- named after the black spots the bubonic form of the plague caused on the skin -- killed indiscriminately regardless of age, sex or level of health because it was so virulent and the European population so immunologically unprepared, DeWitte and Wood wrote.

“The Black Death was highly virulent and undoubtedly killed many otherwise healthy people who would have been unlikely to die under normal-mortality conditions,” they wrote. But people already in poor health were more likely to die, they wrote.

Many scientists think the plague was caused by Yersinia pestis, a bacterial disease spread by fleas from rats. It still kills between 100 and 200 people a year.

The Black Death pandemic thought to have begun in Asia, then spread into the Middle East, Africa and Europe.

“On average, it killed between 30 to 50 percent of affected populations. But we know that there were some areas where mortality was even higher. So there would have been villages that were completely wiped out,” DeWitte said.

Other experts now think the Black Death may have been caused not by bubonic plague but by a viral hemorrhagic fever, similar to the disease caused by the Ebola or dengue viruses.