Viruses are parasitic organisms that live in infected cells and produce virions to disseminate their genes. Most viral proteins have no homologues in modern cells, in contradiction with the traditional view of viruses as pickpockets of cellular genes. This suggests that viral genes essentially originated in the virosphere during replication of viral genomes and/or were recruited from cellular lineages now extinct. Some specific viral proteins are present in viruses infecting members of the three domains of Life, suggesting that viruses are indeed very ancient. In particular, structural analyses of capsid proteins have revealed that at least two types of virions originated independently before the LUCA (the Last Universal Cellular Ancestor). Although several hypotheses have been recently proposed to explain the origin of viruses, the emergence of virions, as a specific mechanism for gene dissemination, remains unexplained.