Think of a URI as the kitty cold. Son having it does not make them immune. They can get it repeatedly. Some are more prone to it than others. My RB Vita use to get at least one per year. And she was never able to fend it off herself, always needed Clavamox to treat it.

Chestnut is sneezing just now, but he seldom gets a cold so I am holding off and hoping it will run its course and he won't need a vet visit. He was sneezing a lot 2 days back, now it is just a few times per day. So I think it is leaving.

The sneezing is the big indicator. I take mine to the vet when it lasts more than 4 days, when the sneezing is non stop, when snot starts shooting out, when the nose gets crusty. Things like that. At that point, I figure it is well settled and not leaving by itself.

Other than Vita, I've never had one cat infect another. As I said above, she was prone to it and if ANYONE in the house got a cold (cat, dog, human) then Vita was sick too.

Kiki may have had the virus, fought it off herself, given it to Desmond, who for whatever reason needed some help getting over it.