From the common cold to COVID-19, viruses have a massive impact on our day-to-day lives, but infections that occurred millions of years ago have shaped our evolution. This is because viral genes have been incorporated into the DNA of the infected host and then passed down the generations, often developing different functions over time. Now, in a study published in Development, Dr. Fumitoshi Ishino, Professor of Molecular Biology at Tokyo Medical and Dental University, Japan, and Dr. Tomoko Kaneko-Ishino, Professor of Molecular Biology at the Tokai University, in Kanagawa, Japan, have discovered that two mouse genes, left behind by a viral infection millions of years ago, have evolved to help defend the brain against new infections.