Thanks Janey, nice to have some reassurance. I can get a bit paranoid about my animals' health and wellbeing - sometimes it can be a bit of a tough one weighing up the potential benefits of a vet visit against the stress it causes them, me - and my bank balance!
I have been doing a bit of internet rifling and it seems that if a cat is a flu carrier, particularly the herpes virus, then they can 'shed' the virus, or have recurrences of symptoms, at times of stress or if they are immunosuppressed. Immunosuppression can include if they've had corticosteroids. Silvie had a steroid jag for her skin thing almost a fortnight ago. I haven't kept the invoice so don't know which drug it was Silvie had to look up the possible side effects, but I do need to go to the vets tomorrow to collect some more arthritis drugs for her so will try and have a chat with the vet or the nurse about it then.