Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia.
Numerous coronaviruses cause respiratory, gastrointestinal, liver, and neurologic diseases in animals. They were first discovered in domestic poultry in the 1930s.
Four coronaviruses, 229E, OC43, NL63, and HKU1, most frequently cause symptoms of the common cold. Rarely, severe lower respiratory tract infections, including bronchiolitis and pneumonia, can occur, primarily in infants, older adults, and immunocompromised people.
Three coronaviruses cause much more severe, and sometimes fatal, respiratory infection in humans than other coronaviruses and have caused major outbreaks of deadly pneumonia in the 21st century:
SARS-CoV-2 was identified as the cause of coronavirus disease 2019 (COVID-19).
MERS-CoV was identified in 2012 as the cause of Middle East respiratory syndrome (MERS).
SARS-CoV-1 was identified in 2003 as the cause of an outbreak of severe acute respiratory syndrome (SARS) that began in China in 2002.
These coronaviruses that cause severe respiratory infection are zoonotic pathogens, which begin in infected animals and are transmitted from animals to people. SARS-CoV-2 has significant person-to-person transmission.
Middle East Respiratory Syndrome (MERS)
Middle East respiratory syndrome (MERS) is a zoonotically severe, acute respiratory illness caused by the MERS coronavirus (MERS-CoV). Symptoms include an influenza-like illness that can progress rapidly to respiratory distress and death. Diagnosis is by polymerase chain reaction testing of respiratory secretions. Treatment is supportive but may involve specific antivirals or interferon therapy.
MERS-CoV is capable of zoonotic spread. Dromedary camels are the primary reservoir. Infected dromedaries are typically asymptomatic; therefore diagnosis in animals is not clinical and relies on laboratory testing (1). MERS-CoV can also spread from person-to-person.
For more information about related veterinary disease, see SARS-CoV-2 Infection in Animals.
MERS-CoV infection was first reported in 2012 in Saudi Arabia (2), but an outbreak in April 2012 in Jordan was confirmed retrospectively (3). Since 2012, worldwide, over 2600 cases of MERS-CoV infection (with over 900 related deaths) have been reported from 27 countries (4); all cases of MERS have been linked through travel to or residence in countries in and near the Arabian Peninsula, with > 80% involving Saudi Arabia. The largest known outbreak of MERS outside the Arabian Peninsula occurred in the Republic of Korea in 2015 (5). The initial outbreak was associated with a traveler returning from the Arabian Peninsula. Cases have also been confirmed in countries throughout Europe, Asia, North Africa, the Middle East, and the United States in patients who were either transferred there for care or became ill after returning from the Middle East. Few cases have been reported since 2019 (4).
The World Health Organization does not recommend any travel or trade restrictions or entry screening related to MERS-CoV and regularly updates risk assessments and technical guidance, including for people traveling to Saudi Arabia for Umrah and Hajj (1).
The median age of patients with MERS-CoV is approximately 52 years, and patients are predominantly male. Infection is usually more severe in older adults and in patients with a preexisting disorder such as diabetes, a chronic heart disorder, or a chronic kidney disorder.
Transmission of MERS-CoV
MERS-CoV may be transmitted from person to person via direct contact, respiratory droplets (particles > 5 micrometers), or aerosols (particles < 5 micrometers).
The reservoir for MERS-CoV is dromedary camels. MERS-CoV can be transmitted to humans from camels via direct contact and possibly via indirect contact by handling or consuming camel-related raw products or secretions (eg, milk, urine) (1).
Many reported cases have involved direct human-to-human transmission in health care settings. If MERS is suspected in a patient, infection control measures must be initiated promptly to prevent transmission in health care settings.
General MERS references
1. World Health Organization (WHO). Middle East respiratory syndrome coronavirus (MERS-CoV). December 11, 2025. Accessed April 22, 2026.
2. Zaki AM, van Boheemen S, Bestebroer TM, Osterhaus AD, Fouchier RA. Isolation of a novel coronavirus from a man with pneumonia in Saudi Arabia. N Engl J Med. 2012;367(19):1814-1820. doi:10.1056/NEJMoa1211721
3. Hijawi B, Abdallat M, Sayaydeh A, et al. Novel coronavirus infections in Jordan, April 2012: epidemiological findings from a retrospective investigation. East Mediterr Health J. 2013;19 Suppl 1:S12-S18.
4. WHO. WHO Health Emergency Programme: Middle East respiratory syndrome coronavirus (MERS-CoV) Dashboard. May 17, 2026. Accessed May 22, 2026.
5. Ki M. 2015 MERS outbreak in Korea: hospital-to-hospital transmission. Epidemiol Health. 2015;37:e2015033. Published 2015 Jul 21. doi:10.4178/epih/e2015033
Symptoms and Signs of MERS
The incubation period for MERS-CoV is about 5 days (ranging between 2 days and 14 days) (1).
Initially, most reported cases involve severe respiratory illness requiring hospitalization, with a case fatality rate of approximately 35% (1, 2). However, some patients who are infected have had mild or no symptoms. People are not thought to be contagious until symptoms develop.
Fever, chills, myalgia, and cough are common. Gastrointestinal symptoms (eg, diarrhea, vomiting, abdominal pain) may also occur. Manifestations may be severe enough to require treatment in an intensive care unit, but the proportion of such cases has declined over time.
Symptoms and signs references
Diagnosis of MERS
Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) testing of upper and lower respiratory secretions and serum
The diagnosis of MERS should be suspected in patients who have an unexplained acute febrile lower respiratory infection with indications of pulmonary parenchymal disease (eg, pneumonia, acute respiratory distress syndrome) who have had any of the following within 14 days of symptom onset (1):
Travel to or residence in an area where MERS has recently been reported or where transmission could have occurred (eg, the Arabian Peninsula)
Contact with a health care facility where MERS has been transmitted
Close contact with a person (or a cluster of people, or dromedary camels) with suspected MERS
MERS should also be suspected in patients who have had close contact with a person with suspected MERS and who have a fever whether they have respiratory symptoms or not.
Testing guidelines are available (2, 3). In the United States, testing is done at state health departments or the CDC. Testing performed outside the United States is typically coordinated via national ministries of health, reported to the World Health Organization (WHO), and displayed on the WHO MERS-CoV dashboard.
Testing should include real-time RT-PCR testing of upper and lower respiratory secretions, ideally taken from different sites and at different times. Serum should be obtained from patients and from all, even asymptomatic, close contacts, including health care professionals (to help identify mild or asymptomatic MERS). Serum is obtained immediately after MERS is suspected or after contacts are exposed (acute serum) and 3 to 4 weeks later (convalescent serum).
In all patients, chest imaging detects abnormalities, which may be subtle or extensive, unilateral or bilateral. In some patients, levels of lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) are elevated and/or levels of platelets and lymphocytes are low. A few patients have acute kidney injury. Disseminated intravascular coagulation and hemolysis may develop.
Diagnosis references
1. WHO. Middle East Respiratory Syndrome Outbreak Toolbox. October 2024. Accessed June 19, 2026.
2. CDC. Laboratory Testing for MERS. December 4, 2024. Accessed December 3, 2025.
3. WHO. WHO Health Emergency Programme: Middle East respiratory syndrome coronavirus (MERS-CoV) Dashboard. May 17, 2026. Accessed June 19, 2026.
Treatment of MERS
Supportive treatment
Treatment of MERS is supportive and includes supplemental oxygen for hypoxemia and careful monitoring of vital signs and organ function (1). Patients who progress to respiratory failure should receive appropriate ventilatory support (eg, mechanical ventilation). Fluid management should be conservative to avoid exacerbating respiratory distress while ensuring adequate perfusion. Complications such as acute respiratory distress syndrome and septic shock should be managed according to established critical care protocols.
For severe cases, ribavirin, interferon alfa-2a, and remdesivir have been used in some centers (2). Limited randomized trial data has shown early initiation (within 7 days of disease onset) of interferon beta-1b plus lopinavir/ritonavir for patients hospitalized with MERS may prevent mortality (3).
To help prevent spread from suspected cases, health care professionals should use standard, contact, and airborne precautions.
There is no currently available vaccine.
Treatment references
1. World Health Organization: Clinical management of severe acute respiratory infection when Middle East respiratory syndrome coronavirus (MERS-CoV) infection is suspected. Interim guidance, Updated January 2019.
2. Arabi YM, Shalhoub S, Mandourah Y, et al. Ribavirin and Interferon Therapy for Critically Ill Patients With Middle East Respiratory Syndrome: A Multicenter Observational Study. Clin Infect Dis. 2020;70(9):1837-1844. doi:10.1093/cid/ciz544
3. Arabi YM, Asiri AY, Assiri AM, et al. Interferon Beta-1b and Lopinavir-Ritonavir for Middle East Respiratory Syndrome. N Engl J Med. 2020;383(17):1645-1656. doi:10.1056/NEJMoa2015294
Severe Acute Respiratory Syndrome (SARS)
Severe acute respiratory syndrome (SARS) is a severe, acute respiratory illness that can be transmitted zoonotically and is caused by the SARS coronavirus (SARS-CoV-1). Symptoms include fever, chills, myalgias, rhinorrhea, and sometimes diarrhea. Diagnosis is clinical, and treatment is primarily supportive.
(See also COVID-19 [SARS-CoV-2 Infection]).
SARS is a severe coronavirus infection that can be transmitted zoonotically. It has a case fatality rate of approximately 15%, but the fatality rate is much higher in older adults (1). SARS is an influenza-like illness that occasionally leads to progressively severe respiratory insufficiency.
SARS-CoV-1 was first detected in the Guangdong province of China in November 2002 and subsequently spread to 28 additional countries (2). During this outbreak, > 8000 cases were reported worldwide, with 774 deaths (approximately a 10% case fatality rate, increased significantly with age, with a mortality rate > 50% in those > 65 years old) (3, 4). The SARS-CoV-1 outbreak was the first time that the United States Centers for Disease Control and Prevention (CDC) advised against travel to a region. This outbreak subsided, and no new cases have been identified since 2004. The immediate source was presumed to be civets that were being sold for food in a live-animal market and had likely been infected via contact with infected bats before they were captured for sale. Bats are frequent reservoir hosts of coronaviruses.
SARS-CoV-1 is transmitted from person to person by close personal contact. It is thought to be transmitted most readily by respiratory droplets produced when an infected person coughs or sneezes.
SARS typically begins with a high fever, chills, myalgias, and mild respiratory symptoms (eg, rhinorrhea). Occasionally, gastrointestinal symptoms such as diarrhea can occur. SARS can progress within a week to a dry cough, hypoxia, or pneumonia. Infrequently, patients with a severe case may require mechanical ventilation.
The diagnosis of SARS is made clinically, and treatment is supportive (5). Coordination of prompt and rigid infection control practices helped rapidly control the 2002 outbreak.
Although no new cases have been reported since 2004, SARS should not be considered eliminated because the causative virus has an animal reservoir from which it conceivably could reemerge.
SARS references
1. World Health Organization. Consensus document on the epidemiology of severe acute respiratory syndrome (SARS). May 17, 2003. Accessed May 21, 2026.
2. Cherry JD: The chronology of the 2002-2003 SARS mini pandemic. Paediatr Respir Rev. 2004;5(4):262-269. doi:10.1016/j.prrv.2004.07.009
3. Centers for Disease Control and Prevention (CDC). Revised U.S. Surveillance Case Definition for Severe Acute Respiratory Syndrome (SARS) and Update on SARS Cases--United States and Worldwide, December 2003. MMWR Morb Mortal Wkly Rep. 2003;52(49);1202-1206.
4. Peiris JS, Yuen KY, Osterhaus AD, Stöhr K. The severe acute respiratory syndrome. N Engl J Med. 2003;349(25):2431-41. doi:10.1056/NEJMra032498
5. Christian MD, Poutanen SM, Loutfy MR, Muller MP, Low DE. Severe acute respiratory syndrome. Clin Infect Dis. 2004;38(10):1420-1427. doi:10.1086/420743
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