An international team led by University College Dublin has identified and sequenced the DNA of the sheep pox virus (SPPV) from 17 ancient pages of medieval manuscripts, including York Gospels and Corpus Glossary. The study, published in Science Advances on 2 September 2026, traces more than 3,500 years of the pathogen's evolution. The disease, highly contagious in small ruminants like sheep and goats, remains a current concern: the European Commission classifies it as a category A disease, requiring immediate eradication measures when outbreaks occur. In Greece, where outbreaks were still reported in 2026, the Commission updated the areas under emergency measures in August. The discovery shows that written documents, traditionally seen as mere carriers of human knowledge, can also preserve biological traces from the past. Parchment, made from animal skin, may hold genetic material from the animal itself, offering a window into its biological history before it was used for writing. By examining documents preserved in the United Kingdom and continental Europe, the team detected SPPV in 17 parchment pages, including one on paper. Combined with archaeological samples, 21 new ancient genomes were reconstructed, offering a broader picture of the virus's past. Among the documents studied, York Gospels, likely created in Canterbury between 990 and 1020 and now housed at York Minster, provided three positive results: two from the original manuscript and one from a later addition. The same genetic traces were found in Corpus Glossary, a document copied around 800 and considered one of the oldest English dictionaries, and in an annotated version of the Epistles of Saint Paul, stored at Trinity College, Cambridge, and dated to the 7th-8th centuries. Out of the 17 pages, only three were made from sheepskin, while ten were from calf and four from goat. This means the presence of SPPV DNA does not necessarily indicate that all the animals used for the pages were infected. The researchers suggest several possibilities: rare cross-species transmission, contamination during the skin preparation process, or the transfer of genetic material between pages. The paper document adds further complexity, as the glue used for binding may have introduced the genetic signal. To understand the virus's origins, the team looked beyond libraries. Sheep teeth from archaeological sites in Russia and Kazakhstan provided much older genetic sequences, dating back to around 1700 BC. This shows that sheep pox has been affecting Eurasian herds for at least 3,700 years. The study also sheds light on the history of capripoxviruses, a group that includes sheep pox, goat pox, and contagious ecthyma viruses. The researchers estimate that the divergence of the main lineages of these viruses occurred between approximately 11,500 and 3,700 years ago, a period that coincides with major changes in pastoral societies, such as the movement of herds and the spread of domesticated animals across Eurasia. Louis L'Hôte, one of the researchers, notes that the genome of the sheep pox virus has remained remarkably stable over the past few millennia. This stability raises questions about earlier genetic changes that might have influenced the virus's adaptation to its host. Sébastien Calvignac-Spencer, who leads the Evolution of Pathogens Department at the Helmholtz Institute for One Health, highlights that having data covering such long periods can help understand how human-driven environmental changes have influenced the spread of diseases. He emphasizes the importance of studying past changes to better predict the emergence of new pathogens. L'Hôte suggests that archives and libraries worldwide may contain genetic traces of ancient animal diseases, offering a unique record of the biological history of livestock and, indirectly, the economies that depended on them.