I’ve read that people in the arctic who died of smallpox and were buried, decades later, upon re-examining their graves live smallpox virus was found. Same for flu of 1918.
The flu thing for 1918...involved them finding individuals in arctic regions who were buried and the research teams were a bit shocked to find that the frozen conditions kept the flu bugs still in a dangerous state. There’s a great book by John M Barry on the flu topic (The Great Influenza). I would highly recommend the book to understand the 1918 episode and what happened.