Poor Harry and you.
I would definitely attribute the bite to him being ill. Infirm and older dogs who have lost some of their senses will sometimes bite. Martha Stewart was bitten on her upper lip by her aging Frenchie and had to have stitches.
If possible, try not to put yourself in between the dogs if they go at it. When my boys have gotten into it, I raise my voice and make a really loud noise to get their attention. I realize this may not work in all situations, but you do risk injury if you physically intervene.