Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link


(Coronavirus Disease 2019; COVID)


The Manual's Editorial Staff

Reviewed/Revised Dec 2023
Get the full details

What is COVID-19?

What causes COVID-19?

COVID-19 is caused by a type of coronavirus called SARS-CoV-2. The virus spreads:

  • Mostly through the air, in droplets that an infected person spreads through a cough, sneeze, singing, or talking

  • Through touching things that infected people have touched and then touching your face

You can spread the virus that causes COVID-19 before you have any symptoms.

What is a coronavirus?

There are hundreds of different coronaviruses that are all related. Most of them infect only animals.

  • Only a few coronaviruses can infect people

  • Some of them cause only colds

  • Three coronaviruses cause very serious illness and can be fatal

What are the symptoms of COVID-19?

Many people with COVID-19 have almost no symptoms.

If you do get symptoms, you'll likely have:

  • Fever

  • Dry cough

  • Trouble breathing

  • Feeling really weak and tired

You may also have:

  • Runny nose

  • Sore throat

  • Loss of your sense of smell and taste

  • Headaches

  • Stomach problems such as nausea, vomiting, and diarrhea

A bad infection goes into your lungs, causing viral pneumonia. That will make you very short of breath. A bad infection can also affect a lot of organs, including your brain, heart, and kidneys.

Is COVID-19 fatal?

You can die from COVID-19. The older you are, the higher your risk of death. Most deaths occur in people over 50 years old. People over 80 are at very high risk. But even children and young adults can die.

How can I tell if I have COVID-19?

You should be tested for COVID-19 if you:

The most common test that a doctor or other healthcare worker does, called a PCR test (polymerase chain reaction test), uses:

  • A swab taken from your nose or throat

The swab is sent to a lab to look for the virus or other evidence of an infection. It can take a few days to get your results.

Antigen tests are a different type of test than a PCR test. Antigen tests can be done at home or in a healthcare setting and uses a swab taken from your nose. They are generally less accurate than PCR tests. But antigen tests can provide rapid results (within 15 minutes).

A blood test can look for antibodies What are the main parts of the immune system? The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including: Germs... read more your body has made to defend against the virus. It takes your body a few weeks to make these antibodies, so this test is not useful when you first get sick. But it can help tell whether you were ever infected.

How is COVID-19 treated?

If you have a mild or moderate case, you'll:

  • Stay home and not go out or go near other people (isolation)

  • Wear a mask so you don't spread the virus

  • Take acetaminophen or ibuprofen for fever and aches

If you have a severe case, with breathing problems, you'll need to go to the hospital. Doctors will:

  • Give you oxygen

  • Sometimes, give you medicines

  • Sometimes, put you on a ventilator (machine to help you breathe)

You may need to be on a ventilator for up to a few weeks.

How can I prevent COVID-19?

The best way to prevent COVID-19 is to be vaccinated COVID-19 Vaccine Coronavirus disease 2019 (COVID-19) vaccines provide protection against COVID-19. COVID-19 is the disease caused by infection with the SARS-CoV-2 virus. There are multiple COVID-19 vaccines... read more . The Centers for Disease Control and Prevention (CDC) recommends that people aged 6 months and older receive the 2023–2024 updated COVID-19 vaccine to protect against serious illness from COVID-19 (see [CDC]: Vaccines for COVID-19).

  • The vaccines are safe and effective

  • You can't get COVID-19 from a COVID-19 vaccine

  • Vaccine effectiveness can decrease over time, so it's important to stay up to date on vaccinations

  • COVID-19 vaccines can cause mild side effects (a sore arm, body aches, headache, fever) for a day or two

  • You should get a COVID-19 vaccine even if you had a COVID-19 infection

In addition to being vaccinated, you should avoid being exposed to the virus. That can be difficult. People who can spread the virus may not have any symptoms, so you can't tell who has it and who doesn't. The CDC recommends additional ways to help avoid the virus based on COVID-19 Hospital Admission Levels. Levels can be low, medium, or high.

You should wear a well-fitting face mask, covering both mouth and nose:

  • When in indoor public places in areas where the COVID-19 Hospital Admission Level is high, regardless of vaccination status

  • If you're at increased risk for severe illness, or you live with or spend time with someone at higher risk, in areas where the COVID-19 Hospital Admission Level is medium or high

  • When sick and around other people

  • When caring for someone who has COVID-19

Consider wearing a face mask when on public transportation (for example, planes, buses, trains) and while indoors at transportation hubs (for example, airports, train stations) regardless of COVID-19 Hospital Admission Level

In addition to being up to date with vaccinations and wearing a mask:

  • Get tested if you have symptoms of COVID-19

  • Avoid crowded places and indoor spaces that do not have fresh air from the outdoors, if you're at higher risk of getting very sick from COVID-19

  • Maintain good social distance (about 6 feet) from other people if you aren't up to date with vaccinations, especially if you're at higher risk of getting very sick with COVID-19

  • If possible, keep 6 feet away from a person who is sick with COVID-19

  • Wash your hands often with soap and water for at least 20 seconds

  • Use an alcohol-based hand sanitizer

  • Avoid touching your eyes, nose, and mouth with unwashed hands

  • Stay home when you're sick

  • Cover a cough or sneeze with a tissue, then throw the tissue in the trash

  • Clean and disinfect frequently touched objects and surfaces using a regular household cleaning spray or wipe

  • Pay attention to your health and watch for symptoms

Quarantine and isolation

Quarantine and isolation measures have been recommended in an attempt to limit the local, regional, and global spread of this outbreak.

Quarantine is meant to separate people who had close contact with a contagious person so they do not infect other people. Currently, the CDC does not advise quarantine for people who have been exposed to Covid-19 but are not infected. See CDC: What to Do If You Were Exposed to COVID-19 for information regarding masking and testing after exposure.

Isolation separates people with confirmed or suspected COVID-19 from those without COVID-19. Regardless of vaccination status, people with COVID-19 symptoms and/or who have a positive COVID-19 test should isolate for at least 5 full days. People who are in isolation should stay home and separated from others, or wear a well-fitting mask when they need to be around others in the home. For an additional 5 days, they should wear a mask when around others both in public and at home. (See also CDC: Isolation and Precautions for People with COVID-19.)

More Information

Drugs Mentioned In This Article

Generic Name Select Brand Names
7T Gummy ES, Acephen, Aceta, Actamin, Adult Pain Relief, Anacin Aspirin Free, Aphen, Apra, Children's Acetaminophen, Children's Pain & Fever , Children's Pain Relief, Comtrex Sore Throat Relief, ED-APAP, ElixSure Fever/Pain, Feverall, Genapap, Genebs, Goody's Back & Body Pain, Infantaire, Infants' Acetaminophen, LIQUID PAIN RELIEF, Little Fevers, Little Remedies Infant Fever + Pain Reliever, Mapap, Mapap Arthritis Pain, Mapap Infants, Mapap Junior, M-PAP, Nortemp, Ofirmev, Pain & Fever , Pain and Fever , PAIN RELIEF , PAIN RELIEF Extra Strength, Panadol, PediaCare Children's Fever Reducer/Pain Reliever, PediaCare Children's Smooth Metls Fever Reducer/Pain Reliever, PediaCare Infant's Fever Reducer/Pain Reliever, Pediaphen, PHARBETOL, Plus PHARMA, Q-Pap, Q-Pap Extra Strength, Silapap, Triaminic Fever Reducer and Pain Reliever, Triaminic Infant Fever Reducer and Pain Reliever, Tylenol, Tylenol 8 Hour, Tylenol 8 Hour Arthritis Pain, Tylenol 8 Hour Muscle Aches & Pain, Tylenol Arthritis Pain, Tylenol Children's, Tylenol Children's Pain+Fever, Tylenol CrushableTablet, Tylenol Extra Strength, Tylenol Infants', Tylenol Infants Pain + Fever, Tylenol Junior Strength, Tylenol Pain + Fever, Tylenol Regular Strength, Tylenol Sore Throat, XS No Aspirin, XS Pain Reliever
Advil, Advil Children's, Advil Children's Fever, Advil Infants', Advil Junior Strength, Advil Migraine, Caldolor, Children's Ibuprofen, ElixSure IB, Genpril , Ibren , IBU, Midol, Midol Cramps and Body Aches, Motrin, Motrin Children's, Motrin IB, Motrin Infants', Motrin Junior Strength, Motrin Migraine Pain, PediaCare Children's Pain Reliever/Fever Reducer IB, PediaCare Infants' Pain Reliever/Fever Reducer IB, Samson-8
quiz link

Test your knowledge

Take a Quiz!