Coronaviruses

Coronaviruses are enveloped, positive-sense single-stranded RNA viruses in the family Coronaviridae and the subfamily Orthocoronavirinae. They infect birds and mammals, including humans. Their genomes are about 26 to 32 kilobases, among the largest for RNA viruses, and encode structural proteins—spike (S), membrane (M), envelope (E), and nucleocapsid (N)—as well as nonstructural proteins. The spike glycoprotein gives the characteristic crown-like appearance on virions.

Genera in Orthocoronavirinae are Alphacoronavirus, Betacoronavirus, Gammacoronavirus, and Deltacoronavirus. Alphacoronaviruses and betacoronaviruses largely infect mammals; gammacoronaviruses and deltacoronaviruses mostly infect birds. In humans, infection is caused by several alphacoronaviruses (e.g., 229E, NL63) and betacoronaviruses (e.g., OC43, HKU1, SARS-CoV-1, MERS-CoV, SARS-CoV-2). Most human infections are mild, but past outbreaks (SARS, MERS) and the ongoing COVID-19 pandemic illustrate potential for severe disease.

Transmission occurs mainly through respiratory droplets and, in some settings, aerosols. Entry begins when the S protein binds a host receptor and is cleaved by proteases such as TMPRSS2, followed by fusion and genome release. Replication occurs in the cytoplasm with production of subgenomic RNAs; virions assemble in the ER-Golgi intermediate compartment and are released by exocytosis. Disease ranges from asymptomatic to severe respiratory illness, depending on virus and host factors.

Evolution is driven by mutation and recombination. Bats are a natural reservoir for many coronaviruses, with spillover to humans or intermediate hosts in some events. Public health measures, surveillance, and vaccination—most notably against SARS-CoV-2—help control disease burden; vaccines for animal coronaviruses are also used in veterinary contexts.