First of all, I don't put any stock in the simplistic estimates that are based on 1918. We're living in a much different world. H5N1 is a different virus.

Some estimates are based on more sophisticated mathematical models. The only problem is, right now critical variables are still unknown.

First variable is known as the lethality rate. That is, what percentage of infected people will bird flu kill?

Right now, based on known victims, it's consistently killing just over half. However, the numbers of patients are still quite small, so that may not be statistically accurate. Plus, many of those patients are receiving good medical care. It may kill a higher percentage of those who don't make it to hospitals.

It's also true that we probably don't know the true number of infected people. Just a few days ago Indonesia announced that it just learned that a little girl who died there last year had bird flu, even though several of her family members were known to have died of it.

If a relatively advanced country isn't getting around to testing an obviously suspect child patient until 8 months after her death -- what's happening in sub-Saharan Africa?

How many more victims are going undetected? That's unknown. On the bright side, it's possible that some or many victims are recovering without being tested so the actual lethality rate might be lower than we think.

If and when H5N1 mutates into a highly contagious form, its lethality rate could go lower. Until it actually happens, we just don't know.

But it is frightening to compare H5N1 to the 1918 flu, because the 1918 flu had a lethality rate of 2.5% -- so if contagious bird flu retains anything like its current apparent lethality rate, it could be much deadlier than 1918 flu. Which would make comparisons between them way over-optimistic.

To measure the spread of a disease, scientists use the contagion rate.

This is -- how many other people on average does one infected person spread the flu to?

Partly this is based on the virus itself, so we just can't know this until it does mutate. However, right now H5N1 is a tough virus -- it can survive for hours outside a human body and in water. This means that you could be infected by an A/H5N1 virus left on a doorknob by someone who opened that door several hours before you.

Overall contagion rate must be affected by population density. Bird flu will infect more people in a crowded Calcutta slum than in rural Wyoming.

And here's a great difference between now and 1918. The world's population is over 6 times higher. But since some areas of the world are so densely crowded, it's possible that chicken flu would spread through them very quickly and kill even more than 6 times as many people as in 1918.

Plus, in 1918 various areas of the world defended themselves by closing themselves off from the outside world. Some places can still do this, but most of the world is much less self-sufficient. Unless you're on an island that supplies its own food and water, you can't isolate yourself from other people.

And even if you can -- there's still the risk of contagion from animals. So your island better not be under a duck migration route. Because duck manure does contain the active virus.

Plus, the world's population may be more susceptible to infection than in 1918. Everybody infected with HIV is at risk, for example.

Bird flu would likely travel around the world more quickly today because we have much more international travel. We go places by jet instead of steam ships.

There's another advance in transportation -- not usually mentioned -- which will affect transmission of bird flu: the automobile. Only a very few people in 1918 had cars, and there was no system of highways.

Now cars and highways connect the entire developed world. And are common and widespread in the developing world.

Of course, riding alone in a car during a lethal flu pandemic is safer than riding in a bus, train or other mass transit vehicle with many other people. Yet if you're the one infected, a car can help you spread the virus farther and faster than you could have in 1918.

There's another risk of death the world's population faces from A/H5N1, which is not being addressed.

That's the risk of the consequences of the pandemic.

If the pandemic causes many deaths the world's economy will be disrupted.

There'll be shortages of food, water, medicine, energy and other necessities. This could last for a year or more, if agriculture is greatly affected.

In many places there's likely to be problems with both law and order and civil unrest.

In a pandemic of any significant seriousness there will death caused by a large number of various groups seeking to take advantage of the situation: ordinary criminals, terrorists, organized crime, renegade groups of soldiers and police, ethnic/racial hate groups, political/religious extremists of all stripes -- and just any old mob looking to loot and kill just because they can get away with it, or to scapegoat some other group for the pandemic.

In some cases there's bound to be organized armed battles and wars between countries and between competing groups within countries.

My personal belief is that there'll be great civil unrest but ultimately society as we know it will prevail -- though the world's map may change in many places.

With central governments weakened by deaths and lack of resources, many disaffected ethnic groups will seize the opportunity to become independent.

On the other hand, many ethnic majorities will seize the opportunity to blame ethnic minorities for the pandemic -- and kill them in riots.

The severity of events will likely fall somewhere between the temporary default of law and order in New Orleans during the immediate aftermath of Hurricane Katrina and the total anarchy of Stephen King's novel THE STAND.

How can anybody predict the number of dead from these events?