Coronavirus

Coronaviruses are a family of RNA viruses that infect mammals and birds, causing a range of illnesses. In humans and birds, these infections affect the respiratory tract and can be as mild as a common cold (though other viruses, especially rhinoviruses, also cause colds) or as severe as SARS, MERS, and COVID-19. In cows and pigs, the viruses lead to diarrhea; in mice, they trigger hepatitis and encephalomyelitis. These viruses belong to the subfamily Orthocoronavirinae, within the family Coronaviridae, order Nidovirales, and realm Riboviria. They are enveloped, with a positive-sense single-stranded RNA genome and a helical nucleocapsid. Their genome is unusually large for an RNA virus, ranging from about 26 to 32 kilobases. A defining feature is the club-shaped spikes covering their surface; under an electron microscope, these spikes resemble a solar corona, which inspired the name.

The term "coronavirus" comes from the Latin *corona* (crown or wreath), itself from Greek *korṓnē* (garland). June Almeida and David Tyrrell coined the name after first observing and studying human coronaviruses. It first appeared in print in 1968, used by a group of virologists in the journal *Nature* to describe the new virus family. The name highlights the virions' appearance under electron microscopy: a fringe of large, bulbous surface projections that look like a halo or solar corona. This shape comes from the viral spike peplomers, proteins on the virus's surface. The International Committee for the Nomenclature of Viruses (later the International Committee on Taxonomy of Viruses) officially accepted *Coronavirus* as a genus name in 1971. As more species were discovered, the genus was split into four genera in 2009: *Alphacoronavirus*, *Betacoronavirus*, *Deltacoronavirus*, and *Gammacoronavirus*. Today, "coronavirus" is the common name for any member of the subfamily Orthocoronavirinae, and as of 2020, 45 species are officially recognized.

Studies from 2021 suggest the most recent common ancestor of coronaviruses may have emerged around 21,000 to 25,000 years ago in East Asia, during the earliest known coronavirus outbreak. Researchers found that 42 coronavirus-specific virus-interacting proteins in East Asian populations were likely selected for during this ancient event, as the viruses drove an adaptive response in the ancestors of East Asians. The four known genera split apart roughly 2,400 to 3,300 years ago into bat and avian coronavirus ancestors. Bat coronaviruses gave rise to the *Alphacoronavirus* and *Betacoronavirus* species that infect mammals, while avian coronaviruses produced the *Gammacoronavirus* and *Deltacoronavirus* species that infect birds.

The first reports of coronavirus infections in animals date to the late 1920s, when an acute respiratory infection appeared in domesticated chickens in North America. Arthur Schalk and M.C. Hawn published the first detailed description in 1931, noting a new respiratory illness in North Dakota chickens. Infected newborn chicks showed gasping and listlessness, with mortality rates between 40 and 90 percent. In 1933, Leland David Bushnell and Carl Alfred Brandly isolated the virus causing the infection, later called infectious bronchitis virus (IBV). Charles D. Hudson and Fred Robert Beaudette first cultivated the virus in 1937, producing the Beaudette strain. In the late 1940s, two more animal coronaviruses were discovered: JHM, which causes brain disease (murine encephalitis), and mouse hepatitis virus (MHV), which causes hepatitis in mice. At the time, researchers did not realize these three viruses were related.

Human coronaviruses were discovered in the 1960s through two separate efforts in the United Kingdom and the United States. In 1961, E.C. Kendall, Malcolm Bynoe, and David Tyrrell at the Common Cold Unit of the British Medical Research Council collected a unique cold virus, designated B814. It could not be cultivated using standard techniques that worked for rhinoviruses, adenoviruses, and other cold viruses. In 1965, Tyrrell and Bynoe succeeded by serially passing the virus through organ cultures of human embryonic trachea, a method introduced by Bertil Hoorn. When volunteers received the virus intranasally, they developed colds, and the virus was inactivated by ether, indicating it had a lipid envelope. Meanwhile, in 1962, Dorothy Hamre and John Procknow at the University of Chicago isolated a different cold virus from medical students, naming it 229E. They grew it in kidney tissue culture, and it also caused colds in volunteers and was inactivated by ether.

In 1967, Scottish virologist June Almeida at St Thomas' Hospital in London, working with Tyrrell, compared the structures of IBV, B814, and 229E using electron microscopy. All three viruses showed similar morphology: a general shape and distinctive club-like spikes. That same year, a research group at the National Institute of Health isolated another member of this new virus group using organ culture, naming one sample OC43 (OC for organ culture). Like B814, 229E, and IBV, OC43 had the same club-like spikes under the microscope. These IBV-like cold viruses were soon shown to be morphologically related to mouse hepatitis virus as well.