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In 1918, the flu pandemic was responsible for an estimated 50 million deaths throughout the world. For decades, researchers did not know why the flu was so fatal, especially for young people between the ages of 20 and 40. Now, according to the researchers of a new study, they might have uncovered the mystery behind the pandemic.

For this study, the team headed by Michael Worobey from the University of Arizona set out to discover why the flu virus killed more than 650,000 Americans who were mainly in their 20s, 30s and 40s. Flu viruses tend to have a greater impact on children and seniors, not young, healthy individuals. The team focused on finding the origins of the virus. They reported that the 1918 strain was created after an existing human H1 flu virus somehow got genetic material from a bird flu virus. The virus, which scientists now know as H1N1, was capable of attacking the body and evading the defenses set up by the immune system.

When the researchers examined influenza strains prior to 1918, they found that people who were born from 1880 to 1900 were mainly exposed to H3N8. Almost all of their blood samples did not contain antibodies for H1, which meant that when the H1 virus hit the population, the people's immune systems were not prepared.

The researchers also found evidence of H1 circulating after 1900. For children born after 1900, they were more exposed to H1, which their immune systems most likely fought off while increasing protection against future H1 virus. For the older generation that was not as affected by the H1 virus in 1918, the researchers found that they also had protection against H1. People born before 1880 were most likely exposed to H1N8.

"We believe that the mismatch between antibodies trained to H3 virus protein and the H1 protein of the 1918 virus may have resulted in the heightened mortality in the age group that happened to be in their late 20s during the pandemic," said Worobey, a professor in UA College of Science's department of ecology and evolutionary biology reported by Fox News.

The study was published in the Proceedings of the National Academy of Sciences.