coronavirus, any of a group (family Coronaviridae, subfamily Coronavirinae) of enveloped single-stranded RNA viruses that have a crownlike or sunlike appearance under an electron microscope due to the presence of spikes on their surface. Coronaviruses infect mammals (including humans) and birds, mainly in the upper respiratory and gastrointestinal tracts. Most only infect a single species or closely related species; a number of coronaviruses can have significant effects on farm animals, especially poultry.

In humans the most prevalent coronaviruses are those that are responsible for varieties of the common cold; coronaviruses are estimated to cause as many as 30% of all colds. More serious are the coronaviruses responsible for SARS (severe acute respiratory syndrome), SARSCoV, and for MERS (Middle East respiratory syndrome), MERSCoV; outbreaks caused by infections with these viruses, which can affect both humans and animals, have resulted in signficant deaths. The SARS outbreak began in China in 2002 and spread globally in 2003; some 9% of those infected died. The first identified MERS outbreak originated in 2012 in Saudi Arabia (the virus appears to be common in dromedary camels); the most significant outbreak outside Saudi Arabia occurred in South Korea in 2015 and was traced to an individual who had visited the Middle East. MERS has killed roughly 40% of those people who are known to be infected, but infections have been relatively rare. Most known cases of MERS have quickly progressed to serious respiratory illness, with fever, cough, pneumonia, shortness of breath, and difficulty breathing. Many MERS patients have also had diarrhea and other gastrointestinal symptoms; kidney failure has sometimes occurred. Older individuals appear to be particularly susceptible. Although there has been person-to-person transmission, MERSCoV (unlike the SARS virus) does not appear to be highly contagious in humans.

Human coronaviruses appear to spread mainly through droplets in the air resulting from coughing or sneezing, or through close contact with an infected person or a contaminated object or surface. There are no vaccines or treatment for coronaviruses; care consists mainly in alleviating symptoms and preventing the viruses' transmission through handwashing and other measures. The first coronavirus, an infectious bronchitis virus of birds, was identified in the 1930s; the first human coronaviruses, associated with the common cold, were identified in the 1960s.