Medieval manuscripts are "biological time capsules" for the deadly sheeppox virus, according to new research. Other archives and libraries worldwide may also contain genetic traces of past disease outbreaks.

Researchers collected samples from manuscripts at Yale University, Cambridge University, Corpus Christi College, University College Dublin, the Lambeth Palace Library, and Trinity College. They found DNA from the sheeppox pathogen on multiple pages, indicating infected animals were used to make parchment and that medieval flocks experienced frequent outbreaks. The virus was also found on calfskin and goatskin, suggesting cross-species infection or contamination during production.

The team also extracted DNA from Bronze Age sheep teeth in Russia and Central Kazakhstan. Comparing sequences, they concluded the sheeppox virus diverged from goatpox and lumpy skin disease viruses between 11,500 and 3,700 years ago, coinciding with animal domestication in Europe. They recovered 21 ancient genomes from 1700 BCE to early modern Europe.

"Parchment making has been considered a vanishing craft, but we are beginning to understand just how much of the past it has retained," said co-author Kevin G. Daly of UCD. The study was published in Science Advances, 2026.