Merck Manual

Please confirm that you are a health care professional

honeypot link

Coronaviruses and Acute Respiratory Syndromes (MERS and SARS)


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Sep 2021| Content last modified Oct 2021
Click here for Patient Education
Topic Resources

Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia.

Numerous coronaviruses, first discovered in domestic poultry in the 1930s, cause respiratory, gastrointestinal, liver, and neurologic diseases in animals. Only 7 coronaviruses are known to cause disease in humans. 

Four of the 7 coronaviruses most frequently cause symptoms of the common cold Common Cold The common cold is an acute, usually afebrile, self-limited viral infection causing upper respiratory symptoms, such as rhinorrhea, cough, and sore throat. Diagnosis is clinical. Handwashing... read more . Coronaviruses 229E, OC43, NL63, and HKU1 cause about 15 to 30% of cases of the common cold. Rarely, severe lower respiratory tract infections, including bronchiolitis and pneumonia, can occur, primarily in infants, older people, and the immunocompromised.

Three of the 7 coronaviruses cause much more severe, and sometimes fatal, respiratory infections in humans than other coronaviruses and have caused major outbreaks of deadly pneumonia in the 21st century:

These coronaviruses that cause severe respiratory infections 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 severe, acute respiratory illness caused by the MERS coronavirus (MERS-CoV).

MERS-CoV infection was first reported in September 2012 in Saudi Arabia, but an outbreak in April 2012 in Jordan was confirmed retrospectively. Through 2019, worldwide, nearly 2500 cases of MERS-CoV infection (with at least 850 related deaths) have been reported from 27 countries; 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. The 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.

Preliminary seroprevalence studies indicate that the infection is not widespread in Saudi Arabia.

The World Health Organization considers the risk of contracting MERS-CoV infection to be very low for pilgrims traveling to Saudi Arabia for Umrah and Hajj. For additional information about pilgrimages to the Middle East, see World-travel advice on MERS-CoV for pilgrimages .

Median age of patients with MERS-CoV is 56 years, and the male:female ratio is about 1.6:1. Infection tends to be more severe in older patients and in patients with a preexisting disorder such as diabetes, a chronic heart disorder, or a chronic renal 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). Person-to-person transmission has been established by the development of infection in people whose only risk was close contact with people who had MERS.

The reservoir of MERS-CoV is thought to be dromedary camels, but the mechanism of transmission from camels to humans is unknown. Most reported cases 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.

Symptoms and Signs of MERS

The incubation period for MERS-CoV is about 5 days.

Most reported cases have involved severe respiratory illness requiring hospitalization, with a case fatality rate of about 35%; however, at least 21% of patients had mild or no symptoms. Fever, chills, myalgia, and cough are common. Gastrointestinal symptoms (eg, diarrhea, vomiting, abdominal pain) occur in about one third of patients. Manifestations may be severe enough to require treatment in an intensive care unit, but recently, the proportion of such cases has declined sharply.

Diagnosis of MERS

  • Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) testing of upper and lower respiratory secretions and serum

MERS should be suspected in patients who have an unexplained acute febrile lower respiratory infection and who have had either of the following within 14 days of symptom onset:

  • Travel to or residence in an area where MERS has recently been reported or where transmission could have occurred

  • Contact with a health care facility where MERS has been transmitted

  • Close contact with a patient who was ill with suspected MERS

MERS should also be suspected in patients who have had close contact with a patient with suspected MERS and who have a fever whether they have respiratory symptoms or not.

The most recent recommendations are available from the Centers for Disease Control and Prevention (MERS: Interim Guidance for Healthcare Professionals).

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 workers (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). Testing is done at state health departments or the Centers for Disease Control and Prevention.

Treatment of MERS

  • Supportive

Treatment of MERS is supportive. To help prevent spread from suspected cases, health care practitioners should use standard, contact, and airborne precautions.

There is no vaccine.

More Information

Severe Acute Respiratory Syndrome (SARS)

Severe acute respiratory syndrome (SARS) is a severe, acute respiratory illness caused by the SARS coronavirus (SARS-CoV).

SARS is much more severe than other coronavirus infections. SARS is an influenza-like illness that occasionally leads to progressively severe respiratory insufficiency.

SARS-CoV was first detected in the Guangdong province of China in November 2002 and subsequently spread to > 30 countries. In this outbreak, > 8000 cases were reported worldwide, with 774 deaths (about a 10% case fatality rate, which varied significantly by age, ranging from < 1% in people ≤ 24 years to > 50% in those ≥ 65 years). The SARS-CoV outbreak was the first time that the Centers for Disease Control and Prevention 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 civet cats, that were being sold for food in a live-animal market and had likely been infected through contact with a bat before they were captured for sale. Bats are frequent hosts of coronaviruses.

SARS-CoV 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.

Diagnosis of SARS is made clinically, and treatment is supportive. Coordination of prompt and rigid infection control practices helped control the 2002 outbreak rapidly.

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.

Click here for Patient Education
NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
Professionals also read
Test your knowledge
Toxoplasmosis is infection with Toxoplasma gondii. The only known hosts of this organism are domestic cats and their relatives. Infection with T. gondii in humans can occur in various ways. Which of the following is the most common mode of infection in humans? 
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID