In 2011, a novel virus isolated from a nasal swab of a 15-week-old pig with influenza-like symptoms in Oklahoma in the USA was found to possess seven (-)ssRNA genomic segments and HEF spike glycoproteins and to share approximately 50% overall aa sequence identity with human ICVs, and it was named C/swine/Oklahoma/1334/2011 (C/OK) [183]. At first, it was suggested to be a new subtype of ICVs due to (i) no cross-reaction of C/OK with human ICVs determined by hemagglutination inhibition assays and (ii) a wider cellular tropism of C/OK than that of a human ICV determined by cell culture studies [183]. In 2016, however, it was determined by the International Committee on Taxonomy of Viruses that this novel influenza virus is distinct from other types, and it was officially classified in a new genus, Deltainfluenzavirus, and so-called influenza D virus (IDV, type (species) D). As shown in Table 1, in addition to pigs, IDVs have been isolated from cattle and have so far been classified into three lineages: D/OK (D/swine/Oklahoma/1334/2011-like viruses), D/660 (D/bovine/Oklahoma/660/2013-like viruses) and D/Japanese, with D/Japanese lineage being further classified into 2 sublineages, D/Yama2016 (D/bovine/Yamagata/10710/2016-like viruses) and D/Yama2019 (D/bovine/Yamagata/1/2019-like viruses), based on phylogenetic and antigenic analyses [73]. Although there has been only serological evidence suggesting that IDV can infect humans [15], the virus may acquire mutations to potentially infect humans and to cause influenza illness in humans.