Villains or heroes? The raison d'être of viruses

Watanabe, Tokiko; Kawaoka, Yoshihiro

Our world is made up of vastly different physical environments and the various organisms that have adapted to live in those environments. The complex interactions between the living and nonliving components of these environments are the basis of the global ecosystem. Various schemes have been proposed to classify living organisms: the one most often used currently defines all living organisms as archaea, bacteria or eukaryotes. Therefore, viruses are not considered living components of the global ecosystem. Given that approximately 10 31 virus particles exist on Earth 56,57 and all of them are parasitic in living organisms, it is not hard to imagine how virus infection might affect the physiological functions of both hosts and the ecosystem. Although 'traditional' virology research tends to focus on pathogenic viruses that cause diseases in their hosts, the recent progress in next-generation sequencing (NGS) technologies and data analyses has enabled us to discover a wide variety of new viruses, some of which do not cause diseases in their hosts. Some obstacles to comprehensive virome analyses remain, such as viral dark matter, which are sequences that originate during virus metagenomics but cannot be aligned to any reference sequences of viruses. 58 Nonetheless, recent viral metagenomic studies using NGS technologies and bioinformatic analyses have identified a large number of viruses in environmental samples, including plants and oceans. 59, 60 Characterisation of these newly identified viruses may provide new insight into the significance of viruses and virus-mediated processes within global ecosystems.