Different from IAVs having a segmented (-)ssRNA genome, CoVs have a nonsegmented (+)ssRNA genome. SARS-CoV and MERS-CoV originated through recombination in bats [267] and cross-species transmission to intermediate hosts, palm civets and dromedary camels, respectively, resulting in transmission to humans. Both palm civet SARSr-CoV and human SARS-CoV and dromedary camel MERS-CoV and human MERS-CoV have 99.8% nt sequence identity [217,268]. In contrast, a nonhuman virus that is almost identical to SARS-CoV-2 has not been identified so far, suggesting no widespread infections of a nearly identical SARS-CoV-2 in natural or intermediate hosts [269]. Based on current data, SARS-CoV-2 might have emerged from a quadruple recombination by which Yunnan bat RaTG13 (96.1% nt identity with SARS-CoV-2) might be its genome backbone [226], Yunnan bat RmYN02 (93.3% nt identity) probably gave the long replicase gene (1ab gene, sharing 97.2% nt identity) [133], pangolin SARS-like-CoV-2/Guangdong might have provided an RBD motif-coding gene (97.4% aa similarity) [231], and an unidentified bat virus might have donated a gene region coding a multibasic (furin, PRRAR motif) cleavage site [133]. Similarly, the 2009 H1N1 pandemic is a quadruple reassortant IAV that acquired gene segments from human IAV (PB1 gene), avian IAV (PB2 and PA genes), classical swine IAV (H1, NP and NS genes) and Eurasian avian-like swine IAV (N1 and M genes) [4].