The 1918 "Spanish flu" was the last great flu pandemic. The death toll was anywhere from 25-40 million worldwide. Out of a world population of 1.8 billion at the time, that's anywhere from 1.3 to 2.2% of the entire world population dead.
It could be happening again.
This time it's the avian "bird flu" it keeps popping up in Asia, and now in far eastern Russia/Siberia.
It's got a rather high mortality rate, something like 80% so far! It's not an exact statistical sample, since the number of people exposed so far has been low, also, there may be other factors like poor hygiene, and medical care in these countries and areas where it has been found. In the 1918 pandemic, secondary pneumonia was often the real killer, which with simple antibiotics and supportive therapy like IV hydration could have saved millions, so a world wide pandemic of "bird flu" might not be as bad, hopefully.
Also, there are no confirmed cases of human-to-human transmission, so far, it has only been caught from birds. Wiping out the poultry stocks of infected areas has worked somewhat.
However, most scientists and virologists think it may only be a matter of time before the virus mutates into a human-to-human contagious form. When it does, all bets are off. If we're lucky, the virus will lose some of it's lethality when it mutates into a human transmissible form. Modern transportation like jet liners and international travel can make quarantines and travel restrictions unworkable. By the time you know there's a problem, it's too late. In the past, when much international travel was by ship, you had weeks to detect sick people and turn them away. Now they're passing customs before the first sniffle.
If we're really, really unlucky, it will still have that 80% mortality rate, and it will spread faster than a vaccine can be made. That would be a true nightmare scenario, because it wouldn't just be 80% dying, and the surviving 20% carrying on (isn't that awful enough?), but there would be tons of ancillary deaths as the survivors died from a breakdown in infrastructure.
People starving because 80% of the farmers and truck drivers are dead. Water supplies failing because 80% of the water workers, power workers are dead. Secondary diseases from all the dead affecting the survivors. Riots, crime, and fire would claim more lives.
That's an extreme scenario however. I'm betting that if bird flu does "break out" and become a big deal, it won't be as bad in the industrialized world, and the mutation to get that human-to-human ability, it might lose some lethality. But I'm still worried. Usually infants and the elderly are the most susceptible, and my wife and I have four children one and under. (Two sets of twins, 11 months apart, if you were wondering how that's possible.)