The epidemic of H5N1 in birds has the potential to be the most significant story of modern times. It could cause concerns about the oil price, Islamic fundamentalism and nuclear war to recede into nothing.
In general, there is a play off in infections between ease of transmission and the seriousness of the infection. A virus that is efficiently transmitted nearly always has a low mortality rate. This is basic Darwinism. A gene that made its carrier susceptible to a high probability of death from the common cold would probably not get passed on at all, and would certainly not prosper.
Of course, some diseases - bubonic plague and smallpox spring to mind - do tend towards the top right hand corner of a graph mapping efficiency of transmission against mortality.
Influenza can become endemic. Every few decades it does, and around a third of a given population become infected. But the mortality rate is low. Usually the number of deaths is tiny, and heavily tilted towards already vulnerable population groups such as the elderly and very young.
H5N1 is not like human influenzas. It is completely on the opposite side of the graph. It has poor transmission - at least in humans - but high mortality - according to WHO it is currently running at 63%. But influenza viruses have a high rate of mutation. What happens if it mutates to the point where it can spread from human to human? According to to WHO, and also to my father-in-law, an internationally renowned immunologist, it is not a matter if, but when. Of course, if it mutates its mortality rate may change. But if it reached endemic proportions while maintaining its current mortality rate it would be the most dramatic event for our species in several centuries. We would be looking at losing 15-20% of the population.
Given current levels of international mobility, it is unlikely that the infection could be limited to a particular area. A billion people would die. And that would not be the end of it. The disruption to the world economy would kill many millions, probably hundreds of millions, more.
I am not saying this because there are precautions we can take. Most of the variables are uncontrollable and unpredictable. I just don't think we should lose sight of this possibility.