Coronaviruses are a group of related viruses in the family Coronaviridae and subfamily Coronavirinae, order Nidovirales. In the subfamily Coronavirinae, coronaviruses include 4 genres: alphacoronavirus, betacoronavirus, gammacoronavirus and deltacoronavirus (Fig. 1 ). Coronaviruses infect a wide variety of hosts including many species of birds, mammals and humans [7]. Alphacoronaviruses and betacoronaviruses circulate among mammals, gammacoronaviruses and deltacoronaviruses infect birds and mammals. Within betacoronaviruses, there are 4 lineages: lineage A contains human coronaviruses HKU1 and OC43, lineage B to which SARS-CoV (Severe Acute Respiratory Syndrome Coronavirus) belongs, lineage C belongs to MERS-CoV (Middle East Respiratory Coronavirus Syndrome) and the lineage D has the bat coronaviruses HKU4 and HKU5 who are close to MERS-CoV.
Fig. 1 Isolates of coronaviruses discussed in this review and their receptors, host and reservoirs. SARS-CoV and SARS-CoV-2 from the lineage B use ACE2 as receptor. MERS-CoV from the lineage C enters into host cells by binding DPP4. SARS-CoV has the masked palm civets as an intermediate host in which the virus has adapted from the Chinese Horseshoe bat reservoir to ultimately infect humans [13]. SARS-CoV-2 has bats and pangolins as natural reservoir and can infect ferrets or domestics animals, with a high susceptibility in cats [14]. MERS-CoV has the origin from bats [15] but maybe this virus had an adaptation through camels before its emergence into human [16].