In humans, coronavirus is usually associated with mild disease, the common cold. Previous emerging novel coronaviruses, such as SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV), which emerged in the Middle East in 2012, were associated with severe and sometimes fatal disease. MERS-CoV was less pathogenic than SARS-CoV, with the most severe infections mainly in individuals with underlying illnesses. Clinically and epidemiologically, the contemporary 2019-nCoV in China seems to resemble SARS-CoV. The genome of 2019-nCoV also appears most closely related to SARS-CoV and related bat coronaviruses.
The infection has now spread widely, with phylogenetic analysis of the emerging viruses suggesting an initial single-locus zoonotic spillover event in November, 2019,
and subsequent human-to-human transmission. The SARS epidemic in 2003 was followed soon after by avian influenza H5N1 in 2006, centred on the Asian continent and Middle East. Other surprising viral zoonoses that have caused serious disease include Nipah encephalitic virus in pigs and humans in southeast and south Asia in 1999–2014, and large-scale Ebola virus epidemics in 2014–16 and 2018–19 in west and central Africa. Taken together, these events ring alarm bells about disease emergence in the 21st century, and the importance of human diseases originating from indiscriminate contacts with infected animals.