The 1918 flu virus has been reconstructed using genes from the tissues of victims of the great pandemic in a reverse genetics process that enables scientists to make fully functioning viruses.
The 1918 virus was indeed different from other known flu viruses.
The virus does something early in infection to trigger a devastating immune response that destroys the lungs in a matter of days. The 1918 virus is also what we see with H5N1 viruses requiring antiviral strategies to both outwit the virus and moderate the host immune response. [story]