Four coronaviruses (HCoV-229E, HCoV-OC43, HCoV-NL63, HCoV-HKU1) are endemic in humans and mainly associated with mild respiratory illnesses; whereas the other two coronaviruses [Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and Middle East Respiratory Syndrome Coronavirus (MERS-CoV)] present as emerging infections causing severe respiratory syndrome. Coronaviruses evolve by accumulation of point mutations and recombination of genomes among different strains or species. Mammalian coronaviruses including those infect humans are evolved from bat coronaviruses. While SARS-CoV and MERS-CoV are genetically closely related to bat coronaviruses, intermediate host(s) is (are) likely to be involved in the emergence and cross-species transmission of these novel human viruses. High prevalence of SARS-like coronaviruses have been found from masked palm civet cats and raccoon dogs collected from markets around the time of outbreaks in humans, but these animals are likely to be a transient accidental host rather than a persisting reservoir. More research is needed to elucidate the ecology of coronaviruses. Vigilance and surveillance should be maintained to promptly identify newly emerged coronaviruses.