Steroid injection and Cat flu

My cat had a steroid injection just over a week ago to treat a possible allergy or stress reaction (his skin was red and sore and scabby.) 6 days later he developed symptoms of cat flu which was confirmed by the vet.   He originally comes from a large group of cats so has likely always been a carrier but is the first time he's had symptoms.   I've always thought steroids affected the immune system. Could the cat flu flair up been caused by the steroids?

Hello!   If a cat is already carrying a virus and has an unspecified dose of an unspecified injection of steroids, which suppress the systemic immune system, then it might be possible that the virus will use that suppression as an opportunity to multiply.  You could ask your vet whether this might have been true of this case.  Steroids are usually given with the intention of stopping the immune system from working excessively in the first place e.g. in the case of an allergy.  Here is a general fact-sheet about them.    https://vethelpdirect.com/vetblog/2020/10/03/why-did-the-vet-prescribe-steroids/