In general, viruses cannot leave behind physical fossils,[1] therefore indirect evidence is used to reconstruct the past.
[2] The most surprising viral fossils originate from non-retroviral DNA and RNA viruses.
Other viral fossils originate from DNA viruses such as hepadnaviruses (a group that includes hepatitis B).
[10] Successful attempts to "resurrect" extinct viruses from the DNA fossils have been reported.
[11] In addition, Pithovirus sibericum was revived from a 30,000-year-old ice core harvested from permafrost in Siberia, Russia.