# Coronavirus
Coronaviruses are a family of enveloped, positive-sense single-stranded RNA viruses that cause respiratory and gastrointestinal disease in mammals and birds, named for the crown-like appearance of the spike proteins on their surface. In humans they range from agents of the common cold to the causes of three major twenty-first-century epidemics and one pandemic: SARS (2002–2004), MERS (from 2012), and COVID-19 (from 2020).
| Key fact | Detail |
|---|---|
| Classification | Order Nidovirales, suborder Cornidovirineae, family Coronaviridae, subfamily Orthocoronavirinae; four genera: alpha, beta, delta, and gamma[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC9948901/pdf/cm2023000006.pdf) • [2](https://jamanetwork.com/journals/jama/fullarticle/2759815) |
| Human species | Seven human coronaviruses: 229E, NL63, OC43, HKU1, SARS-CoV, MERS-CoV, and SARS-CoV-2; only alpha and beta coronaviruses infect humans[3](https://www.cdc.gov/human-coronaviruses/php/types/index.html) • [2](https://jamanetwork.com/journals/jama/fullarticle/2759815) |
| Disease burden | The four endemic HCoVs account for 10% to 30% of upper respiratory tract infections in adults[2](https://jamanetwork.com/journals/jama/fullarticle/2759815) |
| Genome | Among the largest known RNA-virus genomes, though the planarian secretory cell nidovirus has a larger 41.1-kb genome; protected by a viral 3′-to-5′ exoribonuclease (nsp14) that lowers mutation rates to 10−6 to 10−7, versus 10−3 to 10−5 for most RNA viruses[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC7489918/) • [18](https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1007314) |
| Structural proteins | Four: spike (S), envelope (E), membrane (M), and nucleocapsid (N)[5](https://www.nature.com/articles/nrmicro.2016.81) |
| Major receptors | ACE2 for SARS-CoV and SARS-CoV-2, dipeptidyl peptidase 4 for MERS-CoV, aminopeptidase N for many alphacoronaviruses[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC9948901/pdf/cm2023000006.pdf) |
| Animal reservoirs | Bats harbor a wide range of coronaviruses, including SARS-CoV-like and MERS-CoV-like viruses; dromedary camels are the MERS-CoV reservoir[5](https://www.nature.com/articles/nrmicro.2016.81) |
## Structure and classification
Coronaviruses are large, enveloped viruses whose name comes from the spike (S) proteins projecting from the viral envelope, which resemble the spikes of a crown ("corona")[3](https://www.cdc.gov/human-coronaviruses/php/types/index.html). Inside the envelope sits a helical nucleocapsid made of the positive-sense RNA genome encapsidated by the nucleocapsid (N) protein[5](https://www.nature.com/articles/nrmicro.2016.81). The [International Committee on Taxonomy of Viruses](https://www.edgepedia.org/international-committee-on-taxonomy-of-viruses) (ICTV) places coronaviruses in the order Nidovirales, suborder Cornidovirineae, family [Coronaviridae](https://www.edgepedia.org/coronaviridae), and subfamily Orthocoronavirinae[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC9948901/pdf/cm2023000006.pdf). The family divides into four genera, alpha, beta, delta, and gamma, of which only the alpha and beta genera are known to infect humans[2](https://jamanetwork.com/journals/jama/fullarticle/2759815).
The spike protein determines which cells a coronavirus can infect, because it must bind a receptor on the host cell. Most alphacoronaviruses use aminopeptidase N; betacoronaviruses use a range of receptors, including angiotensin-converting enzyme 2 (ACE2) for SARS-CoV and [SARS-CoV-2](https://www.edgepedia.org/sars-cov-2), dipeptidyl peptidase 4 (DPP4) for MERS-CoV, and other molecules for animal betacoronaviruses such as bovine coronavirus and mouse hepatitis virus[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC9948901/pdf/cm2023000006.pdf).
## Replication and proofreading
Coronaviruses use a coding strategy shared across the order Nidovirales: about two-thirds of the viral RNA is translated into two large polyproteins, while the remainder of the genome is transcribed into a nested set of subgenomic mRNAs[5](https://www.nature.com/articles/nrmicro.2016.81). The structural proteins S, E, and M are inserted into the endoplasmic reticulum and travel through the ER-Golgi intermediate compartment, where genomes bound by N protein bud into membranes carrying the structural proteins to form mature virions[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC9948901/pdf/cm2023000006.pdf).
**Proofreading sets coronaviruses apart.** Most RNA viruses replicate with error rates between 10−3 and 10−5, but coronaviruses encode the first known viral RNA proofreading exoribonuclease, the 3′-to-5′ exoribonuclease of nonstructural protein 14 (nsp14-ExoN)[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC7489918/) • [6](https://www.annualreviews.org/doi/10.1146/annurev-virology-031413-085507). This activity lowers coronavirus mutation rates to 10−6 to 10−7, an order of magnitude below most RNA viruses, and mutating the ExoN gene of SARS-CoV or mouse hepatitis virus raises the error frequency back to the range seen in other RNA viruses[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC7489918/). Proofreading is thought to have allowed the expansion of the coronavirus genome, among the largest of RNA viruses, although the planarian secretory cell nidovirus has a larger 41.1-kb genome, and it is a target for antiviral strategies, including combining ExoN inhibitors with nucleoside analogs[6](https://www.annualreviews.org/doi/10.1146/annurev-virology-031413-085507) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC7489918/) • [18](https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1007314).
## History of discovery
The veterinary history of these viruses goes back to at least the 1930s: [avian infectious bronchitis](https://www.edgepedia.org/avian-infectious-bronchitis) in newborn chicks was first described in 1931 by Schalk and Hawn[7](https://www.bmj.com/content/369/bmj.m1547/rr-2). The first human coronavirus was described by the virologist David Tyrrell, director of the [Medical Research Council](https://www.edgepedia.org/medical-research-council)'s Common Cold Research Unit, and his colleague Mark Bynoe in a paper published in *The BMJ* on 5 June 1965; their strain B814 was later recognized as one of the coronaviruses and identified as a cause of the common cold[7](https://www.bmj.com/content/369/bmj.m1547/rr-2). In humans, the first coronaviruses were discovered in the 1960s through isolation in tracheal organ cultures[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7942143/). A classic 1968 publication grouped the human respiratory viruses, mouse hepatitis viruses, and the prototype infectious chicken bronchitis virus together as coronaviruses[9](https://doi.org/10.1016/j.bsheal.2020.11.002).
For decades human coronaviruses were considered inconsequential pathogens causing the common cold. That changed in the twenty-first century, when SARS-CoV and MERS-CoV emerged from animal reservoirs to cause global epidemics with substantial morbidity and mortality[2](https://jamanetwork.com/journals/jama/fullarticle/2759815).
## The seven human coronaviruses
Four common human coronaviruses have been identified since the 1960s and usually cause mild to moderate upper-respiratory illness like the common cold: 229E and NL63 (alphacoronaviruses) and OC43 and HKU1 (betacoronaviruses)[3](https://www.cdc.gov/human-coronaviruses/php/types/index.html). Together they account for 10% to 30% of upper respiratory tract infections in adults[2](https://jamanetwork.com/journals/jama/fullarticle/2759815).
Three other human coronaviruses, all betacoronaviruses identified since the 2000s, can cause severe illness[3](https://www.cdc.gov/human-coronaviruses/php/types/index.html):
- **SARS-CoV** caused an epidemic of severe acute respiratory syndrome during 2002–2004; no cases have been reported since 2004[3](https://www.cdc.gov/human-coronaviruses/php/types/index.html). The World Health Organization announced the end of the SARS epidemic in July 2003[10](https://applications.emro.who.int/imemrf/313/Jundishapur-J-Microbiol-2020-13-6-1-1-8-e103744-eng.pdf).
- **MERS-CoV** was first reported in 2012 and causes Middle East respiratory syndrome[3](https://www.cdc.gov/human-coronaviruses/php/types/index.html). Its case fatality rate is much higher than that of SARS, and dromedary camels are its reservoir, with zoonotic transmission continuing from camels; human-to-human transmission occurs mainly in health care settings[5](https://www.nature.com/articles/nrmicro.2016.81).
- **SARS-CoV-2** was first reported in 2020 and causes coronavirus disease 2019 (COVID-19); the WHO declared the outbreak a pandemic on March 11, 2020[3](https://www.cdc.gov/human-coronaviruses/php/types/index.html) • [10](https://applications.emro.who.int/imemrf/313/Jundishapur-J-Microbiol-2020-13-6-1-1-8-e103744-eng.pdf).
**Transmission differs sharply among them.** SARS-CoV and MERS-CoV patients do not shed large amounts of virus until well after symptom onset, when they are usually already seeking medical care, which helped contain both[5](https://www.nature.com/articles/nrmicro.2016.81). COVID-19, by contrast, shows high viral shedding in the upper respiratory tract early in infection and a high proportion of transmission from pre-symptomatic, asymptomatic, and mildly symptomatic people, characteristics seen in 2009 pandemic influenza but not in SARS or MERS[11](https://onlinelibrary.wiley.com/doi/abs/10.1002/rmv.2171). This difference is a major reason SARS-CoV-2 spread far more extensively than its predecessors[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7942143/).
## Animal reservoirs and spillover
Bats harbor a wide range of coronaviruses, including SARS-CoV-like and MERS-CoV-like viruses[5](https://www.nature.com/articles/nrmicro.2016.81). Zoonotic transmission of SARS-CoV required an incidental amplifying host, palm civets, while dromedary camels are the MERS-CoV reservoir, and serological evidence indicates MERS-CoV-like viruses have circulated in camels for at least three decades[5](https://www.nature.com/articles/nrmicro.2016.81). High-animal-density environments such as farms, markets, kennels, and shelters favor interspecies contact and promote cross-species transmission and disease emergence[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC8514784/). Surveillance programs such as PREDICT and the Global Virome Project aim to identify new coronaviruses with zoonotic potential and track spillover pathways[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC8514784/).
## SARS-CoV-2 and its origins
SARS-CoV-2 emerged in late 2019 with similarity to SARS-CoV but much more extensive transmission[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7942143/). The closest known relative among sequenced viruses is RaTG13, a bat virus with 96.2% similarity to SARS-CoV-2, indicating that the virus likely originated in bats[13](https://time.com/5870481/coronavirus-origins/). A still closer relative, Banal-52, was collected in Laos in 2020 from another bat species of the genus *Rhinolophus*[14](https://www.lemonde.fr/en/science/article/2025/01/01/covid-19-five-years-on-search-for-origin-of-sars-cov-2-remains-fruitless_6736629_10.html). Comparisons of SARS-CoV-2 spike sequences with those of the closest bat-derived sarbecoviruses have revealed insertion and deletion events, including the acquisition of a polybasic furin cleavage site between the S1 and S2 subunits of the spike[15](https://doi.org/10.1038/s41579-023-00878-2).
No definitive proof of the origin of SARS-CoV-2 has been found. Unlike SARS and MERS, where clusters could be traced back to the farms and suppliers involved, no such chain has been established for SARS-CoV-2; the Huanan market in Wuhan may have acted as an amplifier of human cases rather than the point of origin, and both zoonotic and laboratory-related hypotheses remain under discussion[14](https://www.lemonde.fr/en/science/article/2025/01/01/covid-19-five-years-on-search-for-origin-of-sars-cov-2-remains-fruitless_6736629_10.html).
## Ongoing circulation and surveillance
SARS-CoV-2 continues to evolve, and the WHO Coronavirus Network (CoViNet) brings together surveillance programs and reference laboratories to monitor SARS-CoV-2, MERS-CoV, and novel coronaviruses of public health importance[16](https://data.who.int/dashboards/covid19/variants). All three highly pathogenic human coronaviruses, SARS-CoV, MERS-CoV, and SARS-CoV-2, remain on the WHO Blueprint of priority pathogens for research and development[17](https://discovery.ucl.ac.uk/id/eprint/10228275/).
## References
1. [Coronaviral Classification and General Information (2023)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9948901/pdf/cm2023000006.pdf)
2. [Coronavirus Infections—More Than Just the Common Cold, JAMA (2020)](https://jamanetwork.com/journals/jama/fullarticle/2759815)
3. [Human Coronavirus Types, CDC](https://www.cdc.gov/human-coronaviruses/php/types/index.html)
4. [The molecular virology of coronaviruses, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7489918/)
5. [de Wit et al., SARS and MERS: recent insights into emerging coronaviruses, Nature Reviews Microbiology (2016)](https://www.nature.com/articles/nrmicro.2016.81)
6. [Thinking Outside the Triangle: Replication Fidelity of the Largest RNA Viruses, Annual Reviews](https://www.annualreviews.org/doi/10.1146/annurev-virology-031413-085507)
7. [Covid-19: First coronavirus was described in The BMJ in 1965, The BMJ](https://www.bmj.com/content/369/bmj.m1547/rr-2)
8. [Coronavirus entry: how we arrived at SARS-CoV-2, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7942143/)
9. [SARS-CoV-2: A critical review of its history, pathogenesis, transmission, diagnosis and treatment](https://doi.org/10.1016/j.bsheal.2020.11.002)
10. [SARS-CoV, MERS-CoV, SARS-CoV-2: Comparison of Three Emerging Coronaviruses, Jundishapur Journal of Microbiology (2020)](https://applications.emro.who.int/imemrf/313/Jundishapur-J-Microbiol-2020-13-6-1-1-8-e103744-eng.pdf)
11. [The unique features of SARS-CoV-2 transmission, Reviews in Medical Virology (2021)](https://onlinelibrary.wiley.com/doi/abs/10.1002/rmv.2171)
12. [Cross-Species Transmission of Coronaviruses in Humans and Domestic Mammals, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8514784/)
13. [Inside the Global Quest to Trace the Origins of COVID-19, Time (2020)](https://time.com/5870481/coronavirus-origins/)
14. [Covid-19: Five years on, search for origin of SARS-CoV-2 remains fruitless, Le Monde (2025)](https://www.lemonde.fr/en/science/article/2025/01/01/covid-19-five-years-on-search-for-origin-of-sars-cov-2-remains-fruitless_6736629_10.html)
15. [The evolution of SARS-CoV-2, Nature Reviews Microbiology (2023)](https://doi.org/10.1038/s41579-023-00878-2)
16. [COVID-19 variants, WHO COVID-19 dashboard](https://data.who.int/dashboards/covid19/variants)
17. [Respiratory infections due to human common cold coronaviruses, SARS-CoV, MERS-CoV, and SARS-CoV-2, UCL Discovery](https://discovery.ucl.ac.uk/id/eprint/10228275/)
18. [A planarian nidovirus expands the limits of RNA genome size](https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1007314)
---
*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Viruses and acellular agents › Viruses of animals and humans › Coronaviruses*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license