It's only an estimate. It's anybody's guess.
Bird flu have already spread from bird to other species including pigs, human, etc. But it has not acquire the ability to spread from human to human yet.
The flu we have in human can spread from human to human. The main worry we have now is if the bird flu spread to a human and then acquire the ability of the human flu and spread from human to human, then it's a pandemic. It will wipe out a big population. That was what happen in 1917. 50 million ppl gone.
According to a WHO doc, the Spainish flu and the SARS in 2003 were not destroyed, they merely disappear. It's basically God's will. Only thing left for us to pray.
