From "Preparing for the Next Pandemic", New England Journal of Medicine
...An influenza pandemic has always been a great global infectious-disease threat. There have been 10 pandemics of influenza A in the past 300 years. A recent analysis showed that the pandemic of 1918 and 1919 killed 50 million to 100 million people,1 and although its severity is often considered anomalous, the pandemic of 1830 through 1832 was similarly severe — it simply occurred when the world's population was smaller. Today, with a world population of 6.5 billion — more than three times that in 1918 — even a relatively "mild" pandemic could kill many millions of people.
Influenza experts recognize the inevitability of another pandemic. When will it begin? Will it be caused by H5N1, the avian influenzavirus strain currently circulating in Asia? Will its effect rival that of 1918 or be more muted, as was the case in the pandemics of 1957 and 1968? Nobody knows. ...
And some other links
UK says bird flu "as grave a threat as terrorism"
The Flu Wiki
From the Birds