Since I have a family of cats, when one gets a URI, because my place is so small, it's impossible to quarantine them. So I have to hope for the best. When Maya had her URI a month ago, she was put on Amoxicillin. I had another container in the fridge. When Shortie started to sneeze, I immediately started giving it to her. None of my other cats came down with a URI. So I guess that some cats can become immune to URIs. Go figure.