Rhinotracheitis:
It's nearly 80+% certain this cat has 'rhinotracheitis' which is a permanent respiratory virus that causes intermittent bouts of non-lethal airway / sinus disease. Sometimes it's bad enough to effect the eyes, lungs and cause a fever but USUALLY not.
It's okay, but it might need 'periodic' attention
In some cases as long as the cat's eating, you don't even treat.
Sometimes, if the virus's getting in the eyes, you do.
If the cat goes off feed - it likely has the fever from Rhino and needs care.
It is VERY uncommon for a cat to eat, if it has a fever. (But not all cats that don't eat have a fever.)
ORAL LYSEINE paste can help a great deal. New research shows it doesn't work at all. Which is hilarious because while 'science' can't work out an explanation so it's dismissed out of hand - experienced vets see 'quite decent' 80+ % results with it. And it's cheap and easy enough for the owner to get, and do at home.
DocJohnson
For the support of a cat's immune system against recurrences and symptoms of Rhinotracheitis. I've found this PARTICULARLY helpful in the resolution of "dendritic ulcers" on the corneas of the sufferers.
Science has proven that it doesn't work. At all. And the young vets are proud to say so, having never tried it.
For all I know, it's the fat in the formulation or some stupid vitamin but I know that my clients with Rhino-cats who are always getting ulcers or snuffles - find it runs these symptoms off pretty quickly where at one time they were intractable without Interferon or Acyclovir ancestor-drugs.
Viralys Lyseine Gel For Cats