coronavirus, any of a group (family Coronaviridae, subfamily Orthocoronavirinae) of enveloped single-stranded RNA viruses that have a crownlike or sunlike appearance under an electron microscope due to the presence of spikelike or club-shaped projections 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), SARS-CoV; for MERS (Middle East respiratory syndrome), MERS-CoV; and for COVID-19 (coronavirus disease 2019), SARS-CoV-2. Outbreaks caused by infections with these viruses have resulted in signficant deaths, though at varying rates; a lack of immunity in infected persons and of an effective treatment for the diseases, especially in severe cases, has contributed to the deadliness of the outbreaks. In SARS cases, about 9% of those infected died; in MERS cases, roughly 35% have died. The fatality rate of COVID-19, although not yet determined, is significantly less, but the worldwide spread of COVID-19 that began in late 2019 resulted in a far greater number of infections and deaths than in earlier SARS and MERS outbreaks, and attempts to control the disease's spread led to severe economic disruption.

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.

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