Wikipedia
Paelovirology is the study of viruses that existed in the past but are now extinct. In general, viruses cannot leave behind physical fossils Therefore, indirect evidence is used to reconstruct the past. For example, viruses can cause evolution of their hosts, and the signatures of that evolution can be found and interpreted in the present day. Also, viral genetic material that was integrated into another organism has been passed down to our time as a viral fossil. Viral fossil is an informal term for regions of genomes that originate from ancient germline integration of viral genetic material. The scientific term for such regions is endogenous viral element or EVE. EVEs that originate from the integration of retroviruses are known as endogenous retroviruses, or ERVs, and most viral fossils are ERVs. They may be traced to millions of years back, hence the terminology, although strictly speaking, no one has detected a virus in fossils. The most surprising viral fossils originate from non-retroviral DNA and RNA viruses.