H5N1 in a herd of pigs was observed a long time ago by the chief epidemiologist of Vietnam. He determined that each pig had several variants vying for supremacy in the individual animal. Then each dominant variant would contend with the dominant variants of the other pigs to produce a “grand champion”.
He compared this process to being like a small part of a gigantic computer, made of tens of thousands of herds and flocks.
I have seen no comparison of ARDS in lower vs. upper respiratory tract infections, so cannot attest to differences in severity or mortality.
Typically, a lower respiratory tract infection is more severe and deadly, since the inflammation interferes with the ability to breathe and with gas exchange. Some of these patients can be saved with ECMO. The virus can also spread from the lungs to other parts of the body like the brain, where it is very likely to be fatal. I seem to recall that this is what happened with the imported case of H5N1 in Canada a few months ago--the patient died of brain inflammation. (Interesting point: I should look up the type of receptors predominant in the brain, whether they are alpha-2,3 or alpha-2,6.)
Typically, influenzas confined to the upper respiratory tract cause more mild disease and are highly communicable. The ability to breathe is not as impaired. Of course, even seasonal influenzas kill, but the CFR isn't even close to that seen with the avian influenzas.