Fig. 2. Animal origins of human coronaviruses.

Severe acute respiratory syndrome coronavirus (SARS-CoV) is a new coronavirus that emerged through recombination of bat SARS-related coronaviruses (SARSr-CoVs)²⁰. The recombined virus infected civets and humans and adapted to these hosts before causing the SARS epidemic^42,62. Middle East respiratory syndrome coronavirus (MERS-CoV) likely spilled over from bats to dromedary camels at least 30 years ago¹⁰⁰ and since then has been prevalent in dromedary camels. HCoV-229E and HCoV-NL63 usually cause mild infections in immunocompetent humans. Progenitors of these viruses have recently been found in African bats^133,134, and the camelids are likely intermediate hosts of HCoV-229E^134,135. HCoV-OC43 and HKU1, both of which are also mostly harmless in humans, likely originated in rodents. Recently, swine acute diarrhoea syndrome (SADS) emerged in piglets. This disease is caused by a novel strain of Rhinolophus bat coronavirus HKU2, named SADS coronavirus (SADS-CoV)³⁴; there is no evidence of infection in humans. Solid arrows indicate confirmed data. Broken arrows indicate potential interspecies transmission. Black arrows indicate infection in the intermediate animals, yellow arrows indicate a mild infection in humans, and red arrows indicate a severe infection in humans or animals.