Coronavirus, any virus belonging to the family Coronaviridae. Coronaviruses have enveloped virions (virus particles) that measure approximately 120 nm (1 nm = 10−9 metre) in diameter. Club-shaped glycoprotein spikes in the envelope give the viruses a crownlike, or coronal, appearance. The nucleocapsid, made up of a protein shell known as a capsid and containing the viral nucleic acids, is helical or tubular. The coronavirus genome consists of a single strand of positive-sense RNA (ribonucleic acid).

Coronaviridae is generally considered to contain two genera, Coronavirus and Torovirus, which differ in nucleocapsid morphology, the former being helical and the latter being tubular. Coronaviruses are important agents of gastrointestinal disease in humans, poultry, and bovines. In humans, a species known as SARS coronavirus (or Severe acute respiratory syndrome coronavirus) causes a highly contagious respiratory disease that is characterized by symptoms of fever, cough, and muscle ache, often with progressive difficulty in breathing. The virus emerged in humans in 2002; it likely jumped to humans from an animal reservoir, believed to be horseshoe bats. The ability of SARS coronavirus to jump to humans undoubtedly required genetic changes in the virus. These changes are suspected to have occurred in the palm civet, since the SARS virus present in horseshoe bats is unable to infect humans directly (see SARS).

In 2012 another coronavirus capable of causing a severe acute respiratory illness later known as Middle East respiratory syndrome (MERS) was discovered in humans. The first case was found in Saudi Arabia, and others were reported within the following year in France, Germany, Jordan, Qatar, Tunisia, the United Arab Emirates, and the United Kingdom. All confirmed cases were directly or indirectly linked to the Middle East. Of all confirmed cases documented by 2019, roughly one-third had ended in death. The novel MERS coronavirus was similar to other coronaviruses known to have originated in bats and was thought to be passed from bats to other animals before being transmitted to humans. Camels were identified as one possible reservoir for the MERS virus.

In late 2019 a virus apparently closely related to SARS coronavirus emerged in Wuhan, China. The Wuhan coronavirus, later named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), caused an illness known as COVID-19, which was similar to SARS and was being characterized primarily by fever and respiratory symptoms. The virus was likewise highly contagious. By early 2020 it had spread throughout regions of China and had reached the United States and Europe, having been carried by travelers from affected regions. In March the World Health Organization declared the outbreak a pandemic.