Genetic Material of Virus From 1918 Flu Is Found
 http://www.nytimes.com/1997/03/21/us/genetic-material-of-virus-from-1918-flu-is-found.html
One part of the answer is that the Spanish flu virus passed from birds to pigs and then to humans, a mode of transmission that is thought to produce the most dangerous strains of influenza viruses.

1918 killer flu secrets revealed
 http://news.bbc.co.uk/1/hi/health/3455873.stm
"The virus caused three waves of disease. The second of these, between September and December 1918, resulting in the heaviest loss of life...Although most people who were infected with the virus recovered within a week following bed rest, some died within 24 hours of infection."

[Spanish] Flu victim exhumed after 85 years
 http://news.bbc.co.uk/1/hi/england/london/3446389.stm
Lethal secrets of 1918 flu virus
 http://news.bbc.co.uk/1/hi/health/6271833.stm
The preserved body of a flu victim buried in Arctic permafrost was exhumed, and they painstakingly extracted the genetic material needed to work out the structure of the H1N1 virus.

Then, in a maximum "biosafety" facility at Canada's National Microbiology Laboratory they reconstructed a fully functioning virus, and infected macaque monkeys to see what would happen......

"This research provides an important piece in the puzzle of the 1918 virus, helping us to better understand influenza viruses and their potential to cause pandemics."

H1N1 Flu. A Tale of Evolution, Economics, Power Politics and International Law.
 http://www.abovetopsecret.com/forum/thread460552/pg1