How to improve the immune defenses of cats?

Hello, I manage the corporate feline colony and to my great regret we found that a recently deceased cat had FeLV (also called feline leukemia). The cats appear to be in good health (except two) and it would be advisable to raise the immune defenses. Animals infected with this retrovirus are anemic, have kidney failure, chewing problems (due to swollen lymph nodes or glands causing stomatitis), upper respiratory tract problems. I am thinking of feeding black cumin oil. You can kindly indicate if it is okay and any doses to be administered (usually 400 gr cans are provided in various places). Also, at home, I have cats not affected by this problem but some affected by chronic rhinitis. Can you also indicate some specific product for them? Thanks Alberto