Is URI contagious before symptoms appear?

I've got a foster kitten who is having a reccurrance of an upper respiratory infection. She had it when I got her, just got over it when she went to the vet to get spayed, then a week and a half later started showing symptoms again. She is getting treatment again, but I am wondering if she could have been contagious prior to showing symptoms. The other 2 cats I have are not showing symptoms, but my ferret, a recent addition to our furry family, is.

Are you sure its an upper respiratory? Could it be allergies or herpes? Have you given the kitty l-lysine? The only reason I question whether it's an URI is because the others aren't getting it. Ask the vet about allergies or herpes.

Due to the fact that the symptoms come up after stressful situations, I'd lean more towards herpes which manifests itself as an upper respiratory infection.  Stress is a known trigger of upper respiratory infections in my home.  I would start giving your kitty L-Lsyine (500mg twice daily) in addition to whatever other treatment (meds) you're giving.  Once the flare up is over, reduce the lysine to 250 mg twice daily.  You can get the tablets in the supplement section and crush them between two spoons.  Mix in with wet food.  We use the powdered variety of Lysine since we give it to all our kitties daily.

The rescue is leaning towards herpes causing the URI. Right now Gabby is on amoxicillin and some eye drops. Until their vet tech came to see her, I was treating her with terramycin since her first symptom was goopy eyes. I'm getting the L'lysine next week, was going to start her on that once she finishes the antibiotics.

So a URI as a result of herpes isn't contagious then?

IME, and we have had up to 6 cats at one time, when someone gets an upper respiratory, it runs through the house like wildfire. However, when my herpes kitty has a flare-up, no other cats become sick.

Until you know for sure though, I'd quarantine the little guy.

Herpes in cats, aka feline rhinotracheitis, is the main cause of upper respiratory infections. And it is also highly contagious. Not all cats with the virus develop symptoms though (and yes, stress and the strength of the cat's immune system play a big role in that). http://www.aspca.org/pet-care/cat-care/cat-care-herpes.aspx

I've had cats with URI and herpes.  My other cats haven't gotten sick.  Remember, stress is the contributing factor to trigger the infection, so if everyone is stressed, then the likelihood of everyone getting sick increases. 

It's contagious between cats, but AFAIK ferrets can't get cat respiratory viruses.

Ferrets can get the human cold and flu. . .just a little trivia for ya . But I don't think they can get cat "colds". At least mine never have.