Well, I have to hand it to you for at least thinking Henry's death may not have been of natural causes, but I come up blank for that date and that area for a county or two all around Humansville. I even tried to find a skirmish a few days before August 18 on the assumption that the man may have lingered a few days after being shot, but nothing. No, that area suffered from a sudden spell of peace for some days in August 1863, but that would change. Sorry.
I know age 47 is a bit young to die of natural causes, but it does happen.
Did you have some reason to think somebody had it in for Henry Garrison?
Bruce Nichols