But some viruses do leave treir traces in the DNA of infected species. Many viruses replicate in the nucleus of their host's cells and are therefore prone to be incorporated in the cell itself, a process called endogenization. These changes in the, provide a molecular fossil record of past viral invasions and these changes can be researched.
The reconstructed Mesozoic-era virus is remarkably similar to the Hepatitis B Virus that infects people today, the team found. “We’ve had 82 million years of evolution, but they still have the same proteins,” Suh says.
 Suh et al: The genome of a Mesozoic paleovirus reveals the evolution of hepatitis B viruses in Nature Communications - 2013