There’s plenty of infectious agents that can just lay dormant almost indefinitely.
There’s major concern about viruses coming from the melting permafrost in regions like Siberia.
If you want to hear something even more terrifying, prions can last about indefinitely. Chronic wasting disease in deer is particularly bad because a deer might die and its remains will decompose into the earth. But vegetation will later grow, and some of those prions will have contaminated the new vegetation. A new deer will get infected by eating that vegetation, even years later.