5.1  The origin of SARS-CoV-2
A cluster of cases of viral pneumonia of unknown etiology, now known as SARS-CoV-2 pneumonia, occurred in Wuhan, Hubei Province and were reported to health authorities on the December 29, 2019 [60]. This outbreak was associated with a large seafood and animal market [61]. Further investigations are ongoing to determine the origin of the infection. To date, many reports have described possible original and intermediate hosts (Fig. 1 ). Researchers found that the SARS-CoV-2 showed a higher sequence homology to Bat-CoV-RaTG13 that was previously detected in Rhinolophus affinis from Yunnan Province than Bat-SL-CoVZC21 and Bat-SL-CoVZC45, which suggested that the Chinese chrysanthemum bat is the origin of SARS-CoV-2. More recently, the pangolin was believed to be the likely intermediate host due to the fact that there appeared to be approximately 99% sequence homology between SARS-Co V-2 and the consensus sequence derived from >1000 metagenomic samples from the pangolin species [62].
Fig. 1 The origin and inter-host timeline of SARS-CoV-2. All the information is derived from published data. Journal name: (a) Emerging Microbes & Infections, (b) bioRvix, (c) J Med Virol, (d) The Lancet, (e) the New England Journal of Medicine, (f) J Virol, (g) Chin Med J (Engl), (h) https://www.scau.edu.cn/2020/0207/c1300a219015/page.htm. Bat-ZC21: Bat-SL-CoVZC21, GenBank Accession MG772934; Bat-ZC45: Bat-SL-CoVZC45, GenBank Accession MG772933; Bat-RaTG13: yun-nan-Bat-CoV-RaTG13, GISAID accession EPI_ISL_402,131.