While Robert Koch asserted that infectious diseases were caused by bacteria, the existence of viruses —— biological entities even smaller than bacteria —— was not known at that time. “Virus” is a Latin word meaning “venom” and the perception of viruses as something bad has become firmly established thanks to the novel coronavirus that causes COVID-19. However, advances in genome analysis have led to the discovery that endogenous retroviruses and a variety of other viruses are involved in the evolution of many organisms. Viruses that do not fit conventional definitions have also been discovered. A new picture of viruses is emerging.