Genetic researchers are making ground-breaking—and possibly life-saving—discoveries about the 1918 flu pandemic.

WASHINGTON (Reuters) — Researchers have found out what made the 1918 flu pandemic so deadly — a group of three genes that lets the virus invade the lungs and cause pneumonia. …

Yoshihiro Kawaoka of the University of Wisconsin and colleagues at the Universities of Kobe and Tokyo in Japan used ferrets, which develop flu in ways very similar to humans. …

“The 1918 influenza pandemic was the most devastating outbreak of infectious disease in human history, accounting for about 50 million deaths worldwide,” Kawaoka’s team wrote. …

“We wanted to know why the 1918 flu caused severe pneumonia,” Kawaoka said in a statement.

They painstakingly substituted single genes from the 1918 virus into modern flu viruses and, one after another, they acted like garden-variety flu, infecting only the upper respiratory tract.

But a complex of three genes helped to make the virus live and reproduce deep in the lungs. The three genes — called PA, PB1, and PB2 — along with a 1918 version of the nucleoprotein or NP gene, made modern seasonal flu kill ferrets in much the same way as the original 1918 flu, Kawaoka’s team found.