The study led by Yoshihiro Kawaoka of the University of Wisconsin confirms it was the virulence associated with the virus itself that made it so deadly in claiming young adult lives.
Kawaoka found the 1918 flu virus replicates rapidly in macaques, causing respiratory problems, hemorrhage and death. The researchers found the virus accomplishes its work by sending the host's immune system into overdrive, forcing it to attack the body.
The scientists say circulating strains of the bird flu virus H5N1 also appear to affect innate immune responses and cause respiratory problems. That, they said, suggests interventions designed to protect host immunity might help curb the severity of influenza infections.
The research by Kawaoka, Michael Katze and Darwyn Kobasa appears in the current issue of the journal Nature.