What Are Coronaviruses and How Do They Cause Covid-19

Learn more about Coronaviruses, how a new strain, SARS-Cov-2 leads to covid-19 and who is most at risk

For Statistics on global infections deaths and country by country analysis of Covid 19, follow this link. Figures automatically updated daily

Coronaviruses are a large family of viruses that usually cause mild to moderate upper-respiratory tract illnesses, like the common cold. However, three new coronaviruses have emerged from animal reservoirs over the past two decades to cause serious and widespread illness and death.

There are hundreds of coronaviruses, most of which circulate among such animals as pigs, camels, bats and cats. Sometimes those viruses jump to humans—called a spillover event—and can cause disease. Four of the seven known coronaviruses that sicken people cause only mild to moderate disease.

Three can cause more serious, even fatal, disease. SARS coronavirus (SARS-CoV) emerged in November 2002 and caused severe acute respiratory syndrome (SARS). That virus disappeared by 2004. Middle East respiratory syndrome (MERS) is caused by the MERS coronavirus (MERS-CoV). Transmitted from an animal reservoir in camels, MERS was identified in September 2012 and continues to cause sporadic and localized outbreaks.

The third novel coronavirus to emerge in this century is called SARS-CoV-2. It causes coronavirus disease 2019 (COVID-19), which emerged from China in December 2019 and was declared a global pandemic by the World Health Organization on March 11, 2020. To date, at the time of this articles publication, it has infected more than 8 million people globally and killed over 400,000.


In January 2020, a novel coronavirus, SARS-CoV-2, was identified as the cause of an outbreak of viral pneumonia in Wuhan, China. The disease, later named coronavirus disease 2019 (COVID-19), subsequently spread globally. In the first three months after COVID-19 emerged nearly 1 million people were infected and 50,000 died.

The Centers for Disease Control and Prevention (CDC) developed a test to diagnose COVID-19 in respiratory and serum samples. The National Institute of Allergies and Infectious Diseases (NIAID) also is accelerating efforts to develop additional diagnostic tests for COVID-19. These tests are helping facilitate preclinical studies and aid in the development of medical countermeasures.

NIAID COVID-19 research efforts build on earlier research on severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), which also are caused by coronaviruses. MERS is a viral respiratory disease that was first reported in Saudi Arabia in September 2012 and has since spread to 27 countries, according to the World Health Organization. Some people infected with MERS coronavirus (MERS-CoV) develop severe acute respiratory illness, including fever, cough, and shortness of breath.

From its emergence through January 2020, WHO confirmed 2,519 MERS cases and 866 deaths (about 1 in 3). Among all reported cases in people, about 80% have occurred in Saudi Arabia. Only two people in the United States have tested positive for MERS-CoV, both of whom recovered. They were healthcare providers who lived in Saudi Arabia, where they likely were infected before traveling to the U.S., according to the CDC.

Infection with SARS coronavirus (SARS-CoV) can cause a severe viral respiratory illness. SARS was first reported in Asia in February 2003, though cases subsequently were tracked to November 2002. SARS quickly spread to 26 countries before being contained after about four months. More than 8,000 people fell ill from SARS and 774 died. Since 2004, there have been no reported SARS cases. 

Research evidence suggests that SARS-CoV and MERS-CoV originated in bats, and it is likely that SARS-CoV-2 did as well. SARS-CoV then spread from infected civets to people, while MERS-CoV spreads from infected dromedary camels to people. Scientists are trying to determine how SARS-CoV-2 spread from an animal reservoir to people.

Characterizing MERS and COVID-19 Disease

Since the emergence of MERS coronavirus (MERS-CoV) in 2012 and SARS-CoV-2 in 2019, NIAID scientists have advanced their understanding of how the viruses cause disease, focusing on developing animal models of disease and on countermeasures such as diagnostic tests and vaccine candidates. Research by NIAID scientists and others demonstrated that MERS-CoV and SARS-CoV-2 use their spike (S) protein to enter cells and initiate infection. After entering the cell, the viruses delay usual immune system responses, allowing the infection to gain a foothold. By the time the immune system responds, the infection has progressed and becomes much harder to fight.

Scientists also have characterized different strains of MERS-CoV and can determine through tests how those strains affect different animals. They also are studying how the form of the virus—liquid or aerosol—affects its stability. Using a monkey model of MERS, researchers have learned how the virus causes disease in people. For example, scientists at NIAID’s Rocky Mountain Laboratories (RML) demonstrated in a monkey model that clinical signs of MERS appear within 24 hours of infection. They also found that the virus causes disease deep within the lungs, leading to pneumonia. NIAID-funded researchers also have established several mouse models of infection that have been critical in developing MERS-CoV medical countermeasures.

A research group at RML also has developed a rhesus macaque model of SARS-CoV-2 to study COVID-19. The model mimics mild- to-moderate respiratory disease in people, including signs of pneumonia seen in an X-ray; this also is an important diagnostic feature in human patients. They also learned that the virus causes disease in the lungs, and that virus is shed from the nose, throat and rectum in a pattern similar to virus shedding in COVID-19 patients. The scientists are using this model to evaluate treatments and preventive vaccines. 


About Covid-19

COVID‑19 is the new respiratory disease spreading around the world and it is caused by a coronavirus. COVID‑19 is short for “coronavirus disease 2019.”

The virus is thought to spread mainly between people who are in close contact with one another (about 6 feet) and through respiratory droplets produced when an infected person coughs or sneezes.

  • People are most contagious when they are the sickest. But those who don’t have a lot of symptoms can still pass the virus on to others.
  • There is currently no vaccine or cure for COVID‑19 but researchers are working to find one.

What are COVID‑19’s symptoms?

The most common symptoms are very similar to other viruses: fever, cough, and difficulty breathing. Common symptoms of COVID‑19 are changing as more is learned about the disease.

The most common symptoms of COVID-19 are fever, dry cough, and tiredness. Other symptoms that are less common and may affect some patients include aches and pains, nasal congestion, headache, conjunctivitis, sore throat, diarrhea, loss of taste or smell or a rash on skin or discoloration of fingers or toes. These symptoms are usually mild and begin gradually. Some people become infected but only have very mild symptoms.

Most people (about 80%) recover from the disease without needing hospital treatment. Around 1 out of every 5 people who gets COVID-19 becomes seriously ill and develops difficulty breathing. Older people, and those with underlying medical problems like high blood pressure, heart and lung problems, diabetes, or cancer, are at higher risk of developing serious illness.  However, anyone can catch COVID-19 and become seriously ill. 

People of all ages who experience fever and/or  cough associated withdifficulty breathing/shortness of breath, chest pain/pressure, or loss of speech or movement should seek medical attention immediately. If possible, it is recommended to call the health care provider or facility first, so the patient can be directed to the right clinic.

Who is at high risk for COVID‑19?

Everyone is at risk of getting COVID‑19, but some people are at a higher risk of serious illness.

Asthma (moderate-to-severe)

Moderate-to-severe asthma may put people at higher risk for severe illness from COVID-19.

Actions to take

  • Follow your Asthma Action Plan.
  • Keep your asthma under control.
  • Continue your current medications, including any inhalers with steroids in them (“steroids” is another word for corticosteroids).
  • Know how to use your inhaler.
  • Avoid your asthma triggers.
  • If possible, have another member of your household who doesn’t have asthma clean and disinfect your house for you. When they use cleaning and disinfecting products, have them:
    • Make sure that people with asthma are not in the room.
    • Minimize use of disinfectants that can cause an asthma attack.
    • Open windows or doors and use a fan that blows air outdoors.
    • Always follow the instructions on the product label.
    • Spray or pour spray products onto a cleaning cloth or paper towel instead of spraying the product directly onto the cleaning surface (if the product label allows).

Why you might be at higher risk

COVID-19 can affect your respiratory tract (nose, throat, lungs), cause an asthma attack, and possibly lead to pneumonia and serious illness.

Chronic kidney disease being treated with dialysis

Chronic kidney disease being treated with dialysis may increase a person’s risk for severe illness from COVID-19.

Actions to take

  • If you are on dialysis, you should NOT miss your treatments.
  • Contact your dialysis clinic and your healthcare provider if you feel sick or have concerns.
  • Plan to have enough food on hand to follow the KCER 3-Day Emergency Diet Plan for dialysis patients in case you are unable to maintain your normal treatment schedule.

Why you might be at higher risk

Dialysis patients are more prone to infection and severe illness because of weakened immune systems; treatments and procedures to manage kidney failure; and coexisting conditions such as diabetes.

Chronic lung disease

Chronic lung diseases, such as chronic obstructive pulmonary disease (COPD) (including emphysema and chronic bronchitis), idiopathic pulmonary fibrosis and cystic fibrosis, may put people at higher risk for severe illness from COVID-19.

Actions to take

  • Keep taking your current medications, including those with steroids in them (“steroids” is another word for corticosteroids).
  • Avoid triggers that make your symptoms worse.

Why you might be at higher risk

Based on data from other viral respiratory infections, COVID-19 might cause flare-ups of chronic lung diseases leading to severe illness.


Diabetes, including type 1, type 2, or gestational, may put people at higher risk of severe illness from COVID-19.

Actions to take

  • Continue taking your diabetes pills and insulin as usual.
  • Test your blood sugar every four hours and keep track of the results.
  • Make sure that you have at least a two-week supply of your diabetes pills and insulin.
  • Follow the sick day guidelines for people with diabetes.

Why you might be at higher risk

People with diabetes whose blood sugar levels are often higher than their target are more likely to have diabetes-related health problems. Those health problems can make it harder to overcome COVID-19.

Hemoglobin Disorders

Hemoglobin disorders such as sickle cell disease (SCD) and thalassemia may put people at higher risk for severe illness from COVID-19.

Actions to take

  • Ask your healthcare provider about telemedicine or remote healthcare visits, and know when to go to the emergency department.
  • Work with your healthcare provider to manage medications and therapies for your disorder (including hydroxyurea, chelation therapy, blood transfusions, and prescriptions for pain management) and any other health condition you may have (such as diabetes, high blood pressure, and arthritis).
  • Try to prevent vaso-occlusive episodes or pain crises by avoiding possible triggers.

Why you might be at higher risk

Living with a hemoglobin disorder can lead to serious multi-organ complications, and underlying medical conditions (such as heart disease, liver disease, diabetes, iron overload, kidney disease, viral infections, or weakened immune system) may increase the risk of severe illness from COVID-19.


Many conditions and treatments can cause a person to have a weakened immune system (immunocompromised), including cancer treatment, bone marrow or organ transplantation, immune deficiencies, HIV with a low CD4 cell count or not on HIV treatment, and prolonged use of corticosteroids and other immune weakening medications. People who are Immunocompromised

Actions to take

  • If you are immunocompromised, continue any recommended medications or treatments and follow the advice of your healthcare provider.
  • Call your healthcare provider if you have concerns about your condition or feel sick.

Why you might be at higher risk

People with a weakened immune system have reduced ability to fight infectious diseases, including viruses like COVID-19. Knowledge is limited about the virus that causes COVID-19, but based on similar viruses, there is concern that immunocompromised patients may remain infectious for longer than other COVID-19 patients.

Liver disease

Chronic liver disease,  including cirrhosis, may increase risk for serious illness from COVID-19.

Actions to take

  • Take your medications exactly as prescribed.

Why you might be at higher risk

Severe illness caused by COVID-19 and the medications used to treat some severe consequences of COVID-19 can cause strain on the liver, particularly for those with underlying liver problems. People living with serious liver disease can have a weakened immune system, leaving the body less able to fight COVID-19.

People aged 65 years and older

Older adults, 65 years and older, are at higher risk for severe illness and death from COVID-19.

Actions to take

  • Take your medications for any underlying health conditions exactly as prescribed.
  • Follow the advice of your healthcare provider.
  • Develop a care plan that summarizes your health conditions and current treatments.
  • Prepare yourself to stay home for long periods using this checklist.

Why you might be at higher risk

Although COVID-19 can affect any group, the older you are, the higher your risk of serious disease. Eight out of 10 deaths reported in the U.S. have been in adults 65 years or older; risk of death is highest among those 85 years or older. The immune systems of older adults weaken with age, making it harder to fight off infections. Also, older adults commonly have chronic diseases that can increase the risk of severe illness from COVID-19.

People who live in a nursing home or long-term care facility

Many cases of COVID-19 in the U.S. have occurred among older adults living in nursing homes or long-term care facilities

Actions to take

  • Carefully follow your facility’s instructions for infection prevention.
  • Notify staff right away if you feel sick.
  • Ask your caretakers about the actions that are being taken at your nursing home or long-term care facility to protect you and your loved ones, including if and how they are limiting visitors.

Why you might be at higher risk

The communal nature of nursing homes and long-term care facilities, and the population served (generally older adults often with underlying medical conditions), put those living in nursing homes at higher risk of infection and severe illness from COVID-19.

Serious heart conditions

Serious heart conditions, including heart failure, coronary artery disease, congenital heart disease, cardiomyopathies, and pulmonary hypertension, may put people at higher risk for severe illness from COVID-19.

Actions to take

  • Take your medication exactly as prescribed. Continue angiotensin converting enzyme inhibitors (ACE-I) or angiotensin-II receptor blockers (ARB) as prescribed by your healthcare provider for indications such as heart failure or high blood pressure.
  • Make sure that you have at least a two-week supply of your heart disease medications (such as those to treat high cholesterol and high blood pressure).
  • People with hypertension should continue to manage and control their blood pressure and take their medication as directed.

Why you might be at higher risk

COVID-19, like other viral illnesses such as the flu, can damage the respiratory system and make it harder for your heart to work. For people with heart failure and other serious heart conditions this can lead to a worsening of COVID-19 symptoms.

Severe obesity

Severe obesity, defined as a body mass index (BMI) of 40 or above, puts people at higher risk for complications from COVID-19.

 Actions to take

  • Take your medications for any underlying health conditions exactly as prescribed.

Why you might be at higher risk

Severe obesity increases the risk of a serious breathing problem called acute respiratory distress syndrome (ARDS), which is a major complication of COVID-19 and can cause difficulties with a doctor’s ability to provide respiratory support for seriously ill patients. People living with severe obesity can have multiple serious chronic diseases and underlying health conditions that can increase the risk of severe illness from COVID-19.

What about COVID‑19, pregnancy, and newborns?

At this time, pregnant women reportedly have the same COVID‑19 risk as adults who are not pregnant. There is also no clear evidence that a fetus can be infected with COVID‑19 in the womb.

Pregnant women are known to have a higher risk of severe illness when infected with viruses from the same family as COVID‑19 and other viral respiratory infections. This is why pregnant women should take extra precautions to not get COVID‑19. Avoid those who have or who have been exposed to COVID‑19. Wash your hands often. Practice physical distancing. Clean and disinfect frequently touched surfaces daily

  • During this time, it’s normal for care providers to change some prenatal visits to telemedicine.
  • Ask if your delivering hospital has changed any rules, like if you’ll be asked to wear a face covering while giving birth or if there is a new visitor policy.
  • Most babies born to people with COVID‑19 are not affected. However, once the baby is born, it is at risk for infection. New parents should take the same precautions to protect themselves from illness as pregnant women.
  • So far, COVID‑19 has not been detected in breast milk. If you are sick, including with COVID‑19, wear a cloth face covering and wash your hands before each feeding. If pumping milk, do not share your pump with anyone else. Wash your hands before expressing milk or before touching the pump or bottle parts. Follow recommendations for cleaning the pump and parts after each use. If possible, have someone who is not sick feed the baby.
  • Stay in touch with your care team if you feel sick or develop COVID‑19 symptoms.
  • Pregnancy and postpartum can be stressful, and COVID‑19 can add to feelings of uncertainty, stress, anxiety, or depression. Talk with your care provider if you are feeling overwhelmed. Seek help if you are in crisis and feel like you may harm yourself or your baby.

When should I see a doctor?

Knowing when to see a doctor can keep medical care available for those who need it most.

Most mild symptoms can be treated at home. The most up-to-date way to assess your best next steps is to complete our Screening Tool.

  • If you need to see your doctor, call the office before you go. Many physicians’ offices are doing virtual visits. They will tell you what to do based on your location.
  • Testing is limited-availability across the country and is currently being prioritized for healthcare workers, emergency medical service providers, police, and other essential workers, so please consult your doctor for availability in your local area.
  • If you develop emergency warning signs, call emergency services. Emergency warning signs include: severe, constant chest pain or pressure; extreme difficulty breathing; severe, constant lightheadedness; or serious disorientation or unresponsiveness.
Updated on June 11, 2024 2:11 pm
Updated on June 11, 2024 2:11 pm
Updated on June 11, 2024 2:11 pm
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