Right, if the H5N1 finds a host already infected with a readily transmittable human form influenza, and the two viruses comingle in host cells, the chance of a mutant factor developing that has both the lethal aspects of H5N1 and the infectious aspects of the human influenza will be quite high, in one host!!! The H5N1 only needs to modify it's receptor protein to something that human cells membranes have a greater affinity to and the show will begin. It is not a long shot radical mutation, but a very plausible and historically supported event that happens almost annually with current strains.

As far as lethality goes, H5N1, like most other influenza viruses, causes the body to generate cytochyme as an immune response to defend the system. However, in H5N1 currently, the virus overstimulates this response, and a cytochyme storm within the body occurs, which becomes a pathological event that compromises lung tissue. That's why an H5N1 infection is more devastating on young healthy people who's immune systems are working at peak efficiency. The body ends up working against itself. Combine this with the viruses ability to migrate to other organs and cause damage, and that makes this one bug that can do a lot of damage fast, and spread easily.

It would be good if we had the time to see the WHO come up with a broad spectrum vaccine, trying to counter the most likely recombinant variations that could occur. Unfortunately every year we are given vaccines that may or may not precisely target the one virus that will migrate the most. In a way, this little threat is doing us some good, forcing us to focus our efforts on developing more effective treatment processes. Necessity is the mother of invention.