I’m no doctor, but we don’t actually test for viruses directly correct? Instead what we test for is antibodies for said virus?
It’s pretty hard to definitively know if you have been reinfected or just still have antibodies in your system and died of some other respiratory illness after, secondary infections are pretty common from the flu since it beats you down so much.
Viruses are so scary in general because we really haven’t figured out a great way to treat them in hundreds of years of modern medicine.