I realize that making any statement in general is risky business. I was trying not to make my observation about the sick not too broad. I realize there are many unmarked graves of those who dies of disease or of other causes far from home of course. I think however, in general, those who died of disease or accident in places other than Donelson are most likely not buried as Unknowns in Donelson. Those who were killed in action during that time and place of the Donelson siege, who are not known to be buried elsewhere, most likely, but not absolutly of course, might comprise a large percentage of the unknown graves there.
I am not trying to make it so complex, but am trying not to be too general either. I am though, speaking of specific cemeteries and who might logically be among the Unknown dead at that specific cemetery. So my point was meant to be a little more narrowly focused than an overall statement of burials of disease deaths vs battle deaths.
I am speaking of specific battlefields like Donelson. I am generalizing of course, but would it not be somewhat safe (but not 100% accurate of course) to think that among the Unknowns buried at Donelson just may be the men listed as killed there, whose graves are still unaccounted for? I am simply suggesting that list be begun to compile names at battle site cemeteries of the men killed in that specific battle whose graves have never been located. It would then be possible to add those names to the Known burials at cemetery battle sites like Donelson, as possible identities of some of the Unknowns buried there.
The same might be begun for non-battle deaths or hospital deaths like you mention above. For folks to individually contribute to lists being complied of men thought to have died at a given place and time might at least help give some identites to the Unknowns in many different places. of course there is the problem of verification which may never be possible but at least we can begin compile lists of probable Unknowns. The choices are rather narrow. I am fairly sure the three 14th Iowa men that started this thread were not buried Unknown at Cold Harbor for instance. I am simply suggesting their names might appropriately appear on a list as "probable Unknowns at Donelson", and such lists be started for other cemeteries with large Nnknown burials in many other locations.
It is a huge task of course for a few people to tackle alone but countless individual contributions to specific cemetery lists can begin to make a dent I think.