A somwhat technical, but well referenced article, from The Centres for Disease Control and Prevention examines the information we have to date about the 1918 Influenza Pandemic
including -
When and Where Did the 1918 Influenza Pandemic Arise? Were the 3 Waves in 1918–1919 Caused by the Same Virus? If So, How and Why? What Was the Animal Host Origin of the Pandemic Virus? What Was the Biological Basis for 1918 Pandemic Virus Pathogenicity? Why Did the 1918 Virus Kill So Many Healthy Young Adults? and Could a 1918-like Pandemic Appear Again? If So, What Could We Do About It?
This article, which was actually written in 2006 concludes that since it happened once, analogous conditions could lead to an equally devastating pandemic as 'Despite an explosion of data on the 1918 virus during the past decade, we are not much closer to understanding pandemic emergence in 2006 than we were in understanding the risk of H1N1 "swine flu" emergence in 1976.'