The present outbreak of a coronavirus-associated acute respiratory disease called coronavirus disease 19 (COVID-19) has spread to >210 countries, which is rare among acute infectious diseases in recent years, and caused a great threat to global public health [1]. This disease was caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and it is the third documented spillover of an animal coronavirus to humans in only two decades [2,3]. Since the first human coronavirus detected in the 1960s, SARS-CoV-2 is the seventh coronavirus that is known to infect humans (Fig. 1 ) [4]. There are four types of coronaviruses named as α, β, γ and δ, which only α- and β-type contain human pathogenic strains [1,4]. SARS-CoV-2, including severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV), are all β-coronal viruses [3].
Fig. 1 The virus types and symptoms of 7 important pathogenic human coronaviruses.