Quote Originally Posted by Papewaio
Does a virus keep the same severity of illness as it mutates to allow a different vector of dispersion?
I believe that is the very problem that scientists are most concerned about.

Interestingly enough,

Scientists: 1918 Killer Spanish Flu Was a Bird Flu
Scientists who re-created the 1918 Spanish flu say the killer virus was initially a bird flu that learned to infect people. Alarmingly, they find that today's H5N1 bird flu is starting to learn the same tricks.
"These H5N1 viruses are being exposed to human adaptive pressures, and may be going down a similar path to the one that led to the 1918 virus," Taubenberger said in a news conference. "But the H5N1 strains have only a few of these mutations, whereas the 1918 virus has a larger number."
The good news is that the H5N1 flu bug still has a long way to go. The 1918 bug seemed to need several changes in every one of its eight genes. The H5N1 virus is making similar changes but isn't very far along.

"So, for example, in the nuclear protein gene we speculate there are six genes crucial [for human adaptation]," Taubenberger says. "Of those six, three are present in one or another H5N1 strain. But usually there is only one of these changes per virus isolate. That is true of other genes as well. You see four, five, or six changes per gene in the 1918 virus, whereas H5N1 viruses only have one change or so. It shows they are subjected to similar [evolutionary] pressures, but the H5 viruses are early on in this process."
The avian virus adopts its genes from contact with a human virus. The genes "switch" and both apparently mutate.

What is not clear from my readings is what specific genetic material is retained in the swap.

The severity of the symptoms are a product of the virions capability to infect; the more facilitating a virion's genes are towards this end, the more cells that are infected, and ultimately, the greatest the symptoms will be.

The major issue with this mutation is the capability of H5N1 to infect and consequently destory cells; couple that destructive ability with adaptability to a new host and we have the pandemic. We lack the ability to combat it and it is extermely destructive.

IIRC, the human casualties have largerly been a product of deep lung tissue damage. This is unusual in typical influenza.