I am sorry for your loss.We lost Worthy, our big black and white cat earlier this year. He had gastroenteritis that wouldn't clear up properly, even after the vet visited and gave him some injections, and he was very poorly with it. The vet took him, did some tests and found that he had cat aids. There was a chance that with the vet's care, he would have picked up again and that I know that a cat can have some good years after first getting aids but Worthy had been part of the local feral community before we had him and a fighter.
I felt quite strongly that he had already had the good years with us and that this was the start of the last stage where their immune system is very week and any infection can make them very ill. I really didn't want him to go through more illnesses, more trips to the vets, have felt that we have hung on too long with a couple of pets in the past, etc. so I decided to have him put down ;-(