Hi, I have a cat and her 3 kittens, one of which had a dose of cat flu when she was very young. I did try to keep her separate when she was ill, but it was difficult as I only have a small house. None of the others developed the flu, and none of them have had any innoculations ( a contentious point I know but please dont think ill of me ! )
Oh, and the cat who had flu is 14 years old now....and has been to the vet twice in her whole life, once to be spayed and once for the flu! Obviously this is only my experience but I hope this story helps.