In 2020, an invisible germ—a virus—wholly upended our lives. We’re most familiar with the viruses that give us colds or Covid-19. But viruses also cause a vast range of other diseases, including one disorder that makes people sprout branch-like growths as if they were trees. Viruses have been a part of our lives for so long that we are actually part virus: the human genome contains more DNA from viruses than our own genes. Meanwhile, scientists are discovering viruses everywhere they look: in the soil, in the ocean, even in deep caves miles underground.