Overview
Coronavirus disease 2019 (COVID-19), is an illness caused by a virus. The virus is called severe acute respiratory syndrome coronavirus 2 or, more commonly, SARS-CoV-2. It started spreading at the end of 2019 and became a pandemic disease in 2020.
Coronavirus
Coronavirus
Coronaviruses are a family of viruses. These viruses cause illnesses such as the common cold, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS) and coronavirus disease 2019 (COVID-19).
The virus that causes COVID-19 spreads between people in close contact. Most commonly, it spreads through the air in tiny droplets of fluid. Many people with COVID-19 have mild illness or no symptoms. But for older adults and people with certain medical conditions, COVID-19 can lead to the need for care in the hospital or death.
Staying up to date on your COVID-19 vaccine helps prevent serious illness, the need for hospital care due to COVID-19 and death from COVID-19. Other things that may help prevent the spread of this coronavirus include good indoor airflow, physical distancing, wearing a mask in the right setting and good hygiene.
Medicine can limit the seriousness of the viral infection. Most people recover without long-term effects, but some people have symptoms that continue for months.
Symptoms
Typical COVID-19 symptoms often show up 2 to 14 days after contact with the virus.
Symptoms can include:
- Dry cough.
- Shortness of breath.
- Loss of taste or smell.
- Extreme tiredness, called fatigue.
- Digestive symptoms such as upset stomach, vomiting or loose stools, also called diarrhea.
- Pain, such as headaches and body or muscle aches.
- Fever or chills.
- Coldlike symptoms such as congestion, runny nose or sore throat.
People may have only a few symptoms or none at all. People who have no symptoms but test positive for COVID-19 are called asymptomatic. For example, many children who test positive don't have symptoms of COVID-19 illness. People who go on to have symptoms are considered presymptomatic. People in both groups can still spread the COVID-19 virus to others.
Some people may have symptoms that get worse about 5 to 7 days after symptoms start.
Most people with COVID-19 have mild to moderate symptoms. But COVID-19 can cause serious medical complications and lead to death. Older adults and people who already have medical conditions are at greater risk of serious illness.
COVID-19 may be a mild, moderate, severe or critical illness.
- In broad terms, mild COVID-19 doesn't affect the ability of the lungs to get oxygen to the body.
- In moderate COVID-19 illness, the lungs work properly but there are signs that the infection is deep in the lungs.
- Severe COVID-19 means that the lungs don't work correctly, and the person needs oxygen and other medical help in the hospital.
- Critical COVID-19 illness means the lungs and breathing system, called the respiratory system, have failed and there is damage throughout the body.
Rarely, people who catch this coronavirus can develop a group of symptoms linked to inflamed organs or tissues. This illness is called multisystem inflammatory syndrome. When children have this illness, it is called multisystem inflammatory syndrome in children (MIS-C). In adults, the name is MIS-A.
When to see a doctor
People who are at high risk of serious illness may get medicine to block the spread of the COVID-19 virus in the body. Or your healthcare team may plan regular checks to monitor your health.
Get emergency help right away for any of these symptoms in adults:
- Trouble breathing or catching your breath.
- Skin, lips or nail beds that are pale, gray or blue.
- New confusion.
- Trouble staying awake or waking up.
- Chest pain or pressure that is constant.
Get emergency help right away for any of those symptoms in children, as well as the following symptoms:
- Dizziness.
- Dry mouth, crying with few or no tears, or less need to urinate, suggesting dehydration.
- Frequent vomiting or loose stools, called diarrhea, for longer than 24 hours.
- Fever that doesn't go away.
- Trouble staying awake or waking up.
- Being confused or agitated.
This list doesn't include every emergency symptom. If you or a person you're taking care of has symptoms that worry you, get help. Let the healthcare team know about a positive test for COVID-19 or symptoms of the illness.
Causes
COVID-19 is caused by infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
The coronavirus spreads mainly from person to person, even from someone who is infected but has no symptoms. When people with COVID-19 cough, sneeze, breathe, sing or talk, their breath may contain the COVID-19 virus.
The coronavirus carried by the breath of a person with the infection can land directly on the face of a nearby person, after a sneeze or cough, for example. The droplets or particles could also be breathed in by other people if they are close together or in areas with low airflow.
It's possible to get COVID-19 more than once.
- Over time, the body's defense against the COVID-19 virus can fade.
- A person may be exposed to so much of the virus that it breaks through the immune defense.
- As a virus infects a group of people, the virus copies itself. During this process, the genetic code can randomly change in each copy. The changes are called mutations. If the coronavirus that causes COVID-19 changes in ways that make previous infections or vaccination less effective at preventing infection, people can get sick again.
The virus that causes COVID-19 can infect some pets. Cats, dogs, hamsters and ferrets have caught this coronavirus and had symptoms. It's rare for a person to get COVID-19 from a pet.
Risk factors
The main risk factors for COVID-19 are:
- Living with someone who has COVID-19.
- Spending time in places with poor airflow and a higher number of people when the virus is spreading.
- Spending more than 30 minutes in close contact with someone who has COVID-19.
Many factors affect your risk of catching the virus that causes COVID-19. How long you are in contact with the virus, whether the space has good airflow and the activities you're doing in the space all affect the risk. Also, your risk may change based on whether you or others wear masks, whether someone has COVID-19 symptoms, and how close you are to others. Close contact includes sitting next to and talking to other people, for example, or sharing a car or bedroom.
It seems to be rare for people to catch the COVID-19 virus from an infected surface. While the virus is shed in waste, called stool, COVID-19 infection from places such as public bathrooms is not common.
Serious COVID-19 illness risk factors
Some people are at a higher risk of serious COVID-19 illness than others are. This includes people age 65 and older as well as babies younger than 6 months. People in those age groups have the highest risk of needing hospital care for COVID-19.
Serious illness also is a higher risk for people who have weakened immune systems. This may be from a condition such as cancer or HIV. Or from medicines that people may take after organ transplant or during cancer treatment. The risk of serious COVID-19 illness also goes up if someone is pregnant.
Other conditions that raise the risk of serious COVID-19 illness are:
- Heart disease, high blood pressure and heart failure.
- Stroke.
- Diabetes.
- Chronic kidney, liver or lung disease.
- Mood disorders or mental health conditions, such as schizophrenia.
- Sickle cell disease.
- Smoking.
- Body mass index in the obese category.
This list is not complete. Factors linked to a health issue may raise the risk of serious COVID-19 illness too. Examples are living in a group home or lack of access to medical care.
Complications
Complications of severe COVID-19 illness can include:
- Acute respiratory distress syndrome, which is when the body's organs do not get enough oxygen.
- New heart issues or shock caused by the infection or heart conditions.
- Overreaction of the immune system, called the inflammatory response.
- Blood clots.
- Damage to brain function, called encephalopathy.
- Kidney injury.
COVID-19 illness also may worsen medical issues a person already manages.
Post-COVID-19 syndrome
After having COVID-19, some people report that their symptoms continue for months or they develop new symptoms. This syndrome has often been called long COVID. You might hear it called long-haul COVID-19, post-COVID conditions or post-acute sequelae of SARS-CoV-2 (PASC).
Other infections, such as the flu and polio, can lead to long-term illness. But the virus that causes COVID-19 has only been studied since it began to spread in 2019. So research into the specific effects of long-term COVID-19 symptoms continues.
Researchers do think that post-COVID-19 syndrome can happen after COVID-19 illness of any severity.
Getting a COVID-19 vaccine may help prevent post-COVID-19 syndrome.
Prevention
The 2025-2026 COVID-19 vaccines available in the United States include Moderna, Pfizer-BioNTech and Novavax. The vaccines are all approved for people age 65 and older. Each vaccine also is approved for younger people with a health condition that raises the risk of serious COVID-19 illness.
Moderna COVID-19 vaccine 2025-2026 formulas:
- Spikevax. This vaccine is approved for all people age 65 and older and for people ages 6 months to 64 years with a risk of serious COVID-19 illness.
- Mnexspike. This vaccine is approved for all people age 65 and older and for people ages 12 to 64 years with a risk of serious COVID-19 illness.
Pfizer-BioNTech COVID-19 vaccine 2025-2026 formula:
- Comirnaty. This vaccine is approved for all people age 65 and older and for people ages 5 to 64 years with a risk of serious COVID-19 illness.
Novavax COVID-19 vaccine 2025-2026 formula:
- Nuvaxovid. This vaccine is approved for all people age 65 and older and for people ages 12 to 64 years with a risk of serious COVID-19 illness.
Risks linked to serious COVID-19 illness
The risk of getting seriously ill with COVID-19 depends on a range of factors. Information gathered since 2019 shows that the risk of serious COVID-19 illness goes up depending on your age, your health, and where you live or work.
You are more likely to get seriously ill if you:
- Are over age 65.
- Are pregnant.
- Have a weakened immune system due to illness or medicine you take.
- Already have a chronic illness.
- Spend time in places with poor airflow and a higher number of people when the virus is spreading.
- Spend more than 30 minutes in close contact with someone who has COVID-19.
Smoking increases the risk of serious COVID-19 illness. And people with a body mass index in the obese category may have a higher risk as well.
This list is not complete. Not every risk factor for serious COVID-19 illness is known. People of all ages who have no other medical issues have needed hospital care for COVID-19.
Who should get a COVID-19 vaccine?
Getting a COVID-19 vaccine protects against serious illness, the need for hospital care due to COVID-19 and death from COVID-19. Staying up to date with the latest vaccine is most important for people at higher risk. That includes adults over age 65, those with weakened immune systems, people who are pregnant, and people with chronic conditions, such as heart disease, lung disease or obesity.
Serious side effects from the COVID-19 vaccine are extremely rare. Because of that, even healthy adults benefit from the COVID-19 vaccine, which lowers the risk of serious illness at a rate similar to that of the flu vaccine.
Most people are up to date after one dose of the 2025-2026 COVID-19 vaccine from any vaccine maker. Children ages 6 months to 23 months are up to date after two doses of Spikevax, spaced at least four weeks apart.
People with weakened immune systems
Your healthcare team may suggest added doses of COVID-19 vaccine if you have a moderately or seriously weakened immune system. In the U.S., the Food and Drug Administration (FDA) also has authorized the monoclonal antibody pemivibart (Pemgarda) to prevent COVID-19 in some people with weakened immune systems.
Control the spread of the virus
In addition to vaccination, there are other ways to stop the spread of the virus that causes COVID-19.
If you are at a higher risk of serious illness, talk with your healthcare professional about how best to protect yourself. Know what to do if you get sick so you can quickly start treatment.
If you feel ill or have COVID-19, stay home and away from others, including pets, if possible.
In general, make it a habit to:
- Test for COVID-19. If you have symptoms of COVID-19 test for the illness. Or test five days after coming in contact with the virus.
- Help from afar. Avoid close contact with anyone who is sick or has symptoms, if possible.
- Wash your hands. Wash your hands well and often with soap and water for at least 20 seconds. Or use an alcohol-based hand sanitizer with at least 60% alcohol.
- Cover your coughs and sneezes. Cough or sneeze into a tissue or your elbow. Then wash your hands.
- Clean and disinfect high-touch surfaces. For example, clean doorknobs, light switches, electronics and counters regularly.
Try to spread out in crowded public areas, especially in places with poor airflow. This is important if you have a higher risk of serious illness.
Wear a mask when you're inside a crowded place with poor airflow. This is most important when many people in the area are sick with COVID-19 or the flu.
History of infectious disease outbreaks and vaccines timeline.
Learn about the history of major disease outbreaks, epidemics and pandemics, as well as the impact vaccines and research had on many infectious diseases.
Find out more at History of infectious disease outbreaks and vaccines timeline.
Travel and COVID-19
Travel brings people together from areas where illnesses may be present at higher levels. Masks can help slow the spread of viruses that cause respiratory diseases in general, including COVID-19. Masks help the most in places with low airflow and places where you are in close contact with other people. Also, masks can help if the places you travel to or through have high levels of illness.
Masking is especially important if you or a companion have a high risk of serious COVID-19 illness.
Oct. 21, 2025