Villains or heroes? The raison d'être of viruses

Watanabe, Tokiko; Kawaoka, Yoshihiro

The first virus to be identified in humans was the yellow fever virus in 1901 after the discovery of tobacco mosaic virus in 1892 in plants and footand-mouth disease virus in 1898 in animals. 42 Since then, new virus species that infect humans have been identified almost every year. Woolhouse et al. 43 reviewed human viruses that had been described in the literature and recognised by the International Committee on Taxonomy of Viruses (ICTV), and drew a discovery curve for human viruses by plotting the cumulative number of species reported to infect humans; they showed that new species of human viruses have been discovered at a rate of three or four per year. Currently, there are approximately 263 viruses from 25 viral families that are known to be able to infect humans according to the latest ICTV report. 44 In the last a few decades, emerging infectious diseases caused by newly identified viruses, such as Ebola virus, 5-8 SARS and MERS coronaviruses, [10] [11] [12] human immunodeficiency virus (HIV), 9 Nipah virus and Hendra virus, [45] [46] [47] [48] have appeared in human society. Most emerging infectious diseases are zoonotic, caused by viruses that originate in wild animals, such as primates, rodents and bats [49] [50] [51] ; in particular, bats have drawn attention because a recent comprehensive analysis of mammalian hostvirus relationships indicated that bats have a significantly higher proportion of zoonotic viruses than all other mammalian orders. 52 This analysis was part of a study supported by the United States Agency for International Development (USAID) Emerging Pandemic Threats PREDICT programme (https://ohi.vetmed.ucdavis.edu/programs-projects/ predict-project), which was initiated in 2009, working with partners in over 30 countries on global surveillance of viruses to identify and monitor zoonotic pathogens. To date, the PREDICT programme has found over 1100 viruses in animals and humans, including a new Ebola virus and MERSand SARS-like coronaviruses. In 2018, the Global Virome Project (http://www.globalviromeproject. org) was launched, which aims to conduct viral surveillance on an even larger scale than the PREDICT programme. Those involved in this project estimate that~1.67 million yet-to-be-discovered viral species from key zoonotic viral families exist in mammal and avian hosts, and expect that 631 000-827 000 of these unknown viruses have zoonotic potential. 44 In addition to viral surveillance in mammals and avian hosts, Zhang and Holmes's group recently conducted a screen for RNA viruses in diverse host species (more than 186 species other than mammalian and avian hosts) and identified about 200 vertebrate-associated RNA viruses in fishes, reptiles and amphibians. 53 They also found that vertebrate-specific viral families or genera known to infect mammals and birds, including influenza viruses, filoviruses and hantaviruses, are also present in amphibians, reptiles and fish. 53 In addition to the identification of new virus species in diverse hosts, new virus lifestyles have also been found in nature. Suzuki's group, which is one of the groups in our Neo-virology research consortium, recently reported a new virus lifestyle exhibited by two RNA viruses: a double-stranded (ds) RNA virus (yado-nushi virus 1, YnV1) and a positive-sense, single-stranded [(+)ss] RNA virus (yado-kari virus 1, YkV1) in a phytopathogenic fungus, Rosellinia necatrix. 54 They found that the (+)ssRNA virus (YkV1), which does not have its ow