They can make sure that the infection hasn't come back.
By the way, I've had a cat
that got sooooo stressed by a new cat
that she stopped eating and washing herself. I had to give the new cat to my mom. I wasn't a happy camper about it, but I didn't want to put my older kitty through the stress. As soon as the new cat left, she was her normal self again. That is proof to me what stress can do to an animal. I think that if you keep them separated, you'll have wonderful results.