Sounds like you're doing everything right! I had URI go through all my fosters several months ago when I brought a new foster in from the shelter. She was isolated in my spare bathroom and I took every precaution to disinfect, but it still managed to spread. The good news is that Shiloh, my own cat, never got it at all. She's always been current on her vaccinations and doesn't have a compromised immune system. Hopefully your cats will be okay too, and if it does hit them, it won't be as bad as Popeye's feeling it right now. You might want to run a hot shower a few times a day and put Popeye in the bathroom so the steam can help clear him up. Thats what I did for my cats and it really helped. They had the URI so bad, they were open mouth breathing and everything

I'm glad you know what is wrong now. I hope Popeye has a speedy recovery!