The flu epidemic of 1918 ranks with the Black Death of the Middle Ages as one of the deadliest contagions of all times. The virus swept across the Earth, killing an estimated 20 million people in little over a year. In the United States, more than half a million people died from the illness between September 1918 and June 1919. To this day, no one knows why the virus was so deadly.

Called the Spanish flu, the illness started with aches and fever. As the disease progressed, its victims' faces turned dark, the soles of their feet blackened and they coughed blood. In days, sometimes hours, those infected essentially drowned, their lungs heavy, sodden and engorged with a thin, bloody liquid.

Nearly everyone caught the flu in 1918 in some form; 2.5 percent of its victims died, making the strain 25 times more deadly than any flu before or since.

The flu left behind many questions: why was it so deadly, why were young, apparently healthy people particularly affected, and why did it never reappear? Researchers continue to look for those answers, and two studies published this week in Science magazine shed new light on the killer strain's origins. Analysis of a protein coating the virus suggests it started out as an avian virus. Another study, in the Proceedings of the National Academy of Sciences, shows that combining pieces of the 1918 strain with a mouse flu virus results in a very lethal flu.

As NPR's Richard Knox reports, the findings trouble health officials who worry a similar scenario is developing in Asia. The region is currently battling a massive avian flu outbreak, which has infected hundreds of millions of birds and killed 16 people. So far, evidence suggests the virus isn't easily spread among humans. But health officials fear the bird virus might combine with a human flu virus, unleashing another potentially uncontrollable pandemic among people.

Copyright 2022 NPR. To see more, visit https://www.npr.org.