I had a cat with a VERY serious URI; at one point he stopped eating and became emaciated. The lysine I gave him for the URI (which helped dramatically) wasn't working and too hard on an empty stomach. Finally I read to give tuna out of a can... esp. the juice. It worked! He couldn't resist the smell and started eating. He would eat a bit, turn away, then I would say "keep eating" and put him back in front of his plate. Then praise him for eating and stroke him. Pretty soon he got his appetite back and I didn't have to coax him anymore. Between the tuna, putting him back in front of the food, and rewarding with praise and affection (and after he started eating, lysine plus fresh air did amazing things for the URI) he gained all his weight back and was almost healthy. But the great outdoors that was getting him healthy also seemed to have dangers as he didn't come back one evening: could have been coyote, mean dog or ignorant tourist assuming he was a stray (no collar).
Now I'm using ACV for my new kitten who struggles with eye infectons. I swipe diluted and sometimes full strength across his CLOSED eye and by morning his eye is wide open again. I need to try it in his food and more often for a few days to kick the thing, I think.