Flu (influenza) - symptoms and causes (2023)


The flu is an infection of the nose, throat, and lungs. It spreads easily.

This article discusses types of influenza A and B. Another type of influenza is swine flu (H1N1).

alternative names

Complaint A; Influenza B; Oseltamivir (Tamiflu) - complaint; Zanamivir (Relenza) - complaint; cow - complaint


The flu is caused by an influenza virus.

Most people get the flu when they breathe in small airborne droplets from a cough or sneeze from someone who has the flu. You can also get the flu if you touch something that has the virus on it and then touch your mouth, nose, or eyes.

People often confuse colds and the flu. They are different, but you may have some of the same symptoms. Most people catch a cold several times a year. By contrast, people generally only get the flu once every few years.

Sometimes you can catch a virus that makes you vomit or have diarrhea. Some people call it the "stomach flu." This is a misleading name because this virus is not the real flu. The flu mainly affects the nose, throat and lungs.


Flu symptoms often start quickly. You may start to feel sick 1 to 7 days after coming in contact with the virus. Most often, symptoms appear within 2 to 3 days.

The flu spreads easily. It can affect a large group of people in a very short period of time. For example, students and co-workers often get sick within 2-3 weeks after the flu arrives at school or the workplace.

The first symptom is a fever between 102°F (39°C) and 106°F (41°C). An adult usually has a lower fever than a child.

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Other common symptoms include:

  • pains in the body
  • Shaking chills
  • Dizziness
  • Flushed face
  • Headache
  • Lack of energy
  • nausea and vomiting

The fever and aches start to go away between days 2 and 4. But new symptoms appear, including:

  • Dry cough
  • Increased symptoms affecting breathing.
  • Coryza (clear and watery)
  • sneeze
  • Sore throat

Most symptoms disappear in 4 to 7 days. The cough and feeling tired can last for weeks. Sometimes the fever returns.

Some people may not feel like eating.

The flu can make asthma, breathing problems, and other long-term (chronic) diseases and conditions worse.

Exams and Tests

Most people do not need to see a doctor when they have flu symptoms. This is because most people are not at risk of a severe case of the flu.

If you are very sick with the flu, you may want to see your doctor. People at high risk of complications from the flu may also want to see a doctor if they come down with the flu.

When many people in an area have the flu, a provider can make a diagnosis after learning about your symptoms. No other tests are required.

There is a test to detect the flu. It is done by rubbing the nose or throat. Most of the time, test results are available very quickly. The test can help your doctor prescribe the best treatment.



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Acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) help bring down a fever. Sometimes providers suggest that you use both types of medicines. DO NOT use aspirin.

The fever does not have to drop to a normal temperature. Most people feel better when the temperature drops 1 degree.

Over-the-counter cold remedies may improve some of your symptoms. Cough drops or throat sprays will help with a sore throat.

You will need a lot of rest. Drink lots of fluids. DO NOT smoke or drink alcohol.


Most people with milder symptoms feel better in 3 to 4 days. They do not need to see a provider or take antiviral drugs.

Providers can give antiviral medicines to people who get very sick with the flu. You may need these medicines if you are more likely to have complications from the flu. The following health problems can increase your risk of getting sicker with the flu:

  • Lung disease (including asthma)
  • Heart conditions (except high blood pressure)
  • Kidney, liver, nerve and muscle diseases
  • Blood disorders (including sickle cell disease)
  • Diabetes
  • A weakened immune system due to disease (such as AIDS), radiation therapy, or certain drugs, including chemotherapy and corticosteroids
  • Other long-term medical problems

These medicines can shorten the duration of symptoms by about 1 day. They work best if you start taking them within 2 days of the first symptoms.

Children who are at risk of a severe case of the flu may also need these medicines.

Outlook (forecast)

Millions of people in the United States get the flu each year. Most people get better in a week or two, but thousands of people with the flu develop pneumonia or a brain infection. They need to stay in the hospital. About 36,000 people in the United States die each year from flu problems.

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Anyone, at any age, can have serious complications from the flu. Those most at risk include:

  • People over 65 years of age
  • Children under 2 years
  • Pregnant women over 3 months
  • Anyone living in a long-term care facility
  • Anyone with chronic heart, lung, or kidney problems, diabetes, or a weakened immune system

Possible complications

Complications can include:

  • Pneumonia
  • Encephalitis (infection of the brain)
  • Meningitis
  • seizures

When to contact a medical professional

Call your doctor if you have the flu and think you are at risk for complications.

Also, call your doctor if your flu symptoms are severe and self-treatment isn't working.


You can take steps to avoid getting or spreading the flu. The best step is to get a flu shot.

Flu (influenza) - symptoms and causes (1)

If you have the flu:

  • Stay in your apartment, dormitory, or house for at least 24 hours after your fever is gone.
  • Wear a mask if you leave the room.
  • Avoid sharing food, utensils, vases or bottles.
  • Use hand sanitizer frequently throughout the day and always after touching your face.
  • Cover your mouth with a tissue when coughing and throw it away after use.
  • Cough into your sleeve if a tissue is not available. Avoid touching your eyes, nose and mouth.

The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months and older receive the flu vaccine. Children 6 months through 8 years of age may need 2 doses during a single flu season. Everyone else only needs 1 dose each flu season. For the 2019-2020 season, the CDC recommends the use of both the influenza vaccine (inactivated influenza vaccine, or IIV) and the recombinant influenza vaccine (RIV). Nasal spray influenza vaccine (live attenuated influenza vaccine or LAIV) can be given to healthy, non-pregnant people 2 through 49 years of age.


Aoki fiscal year. Antiviral drugs for influenza and other respiratory virus infections. In: Bennett JE, Dolin R, Blaser MJ, eds.Mandell, Douglas, and Bennett Principles and practice of infectious diseases.9th ed. Philadelphia, Pennsylvania: Elsevier; 2020: chapter 45.

(Video) Symptoms of flu (influenza)

Centers for Disease Control and Prevention website. Influenza VIS inactivated.www.cdc.gov/vaccines/hcp/vis/vis-statements/flu.html🇧🇷 Updated August 15, 2019. Accessed October 19, 2020.

Centers for Disease Control and Prevention website. Influenza SIV live, intranasal.www.cdc.gov/vaccines/hcp/vis/vis-statements/flulive.html🇧🇷 Updated August 15, 2019. Accessed October 19, 2020.

Centers for Disease Control and Prevention website. What you should know about antiviral drugs for the flu.www.cdc.gov/flu/treatment/whatyoushould.htm🇧🇷 Updated August 31, 2021. Accessed March 11, 2022.

Havers FP, Campbell AJP. Virus de la gripe. En: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds.Nelson Treatise on Pediatrics.21st edition Philadelphia, Pennsylvania: Elsevier; 2020: chapter 285.

Ison MG, Hayden FG. Queja. En: Goldman L, Schafer AI, eds.Medicina Goldman-Cecil.26th edition. Philadelphia, Pennsylvania: Elsevier; 2020: chapter 340.

Treanor JJ. Influenza viruses, including bird flu and swine flu. In: Bennett JE, Dolin R, Blaser MJ, eds.Mandell, Douglas, and Bennett Principles and Practice of Infectious Diseases🇧🇷 9th ed. Philadelphia, Pennsylvania: Elsevier; 2020: chapter 165.

version information

  • Last review on 8/13/2020
  • Linda J. Vorvick, MD, Clinical Associate Professor, Department of Family Medicine, UW Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and A.D.A.M. Editorial team. Editorial update 03/11/2022.

Flu (influenza) - symptoms and causes (2)ADAM, Inc. is accredited by URAC, also known as the American Commission on Health Care Accreditation (www.urac.org). URAC's accreditation program is the first of its kind and requires compliance with 53 quality and responsibility standards, verified by an independent audit. ADAM. was one of the first to win this important distinction for online health information and services. Learn more about the editorial process of A.D.A.M. ADAM. it is also a founding member of Hi-Ethics (www.hiethics.com) and subscribes to the principles of the Health on the Net foundation (www.hon.ch).

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What is the main cause of flu-like symptoms in influenza? ›

Flu-like symptoms may be caused by viral infections or other illnesses that are not caused by the influenza virus. They may also occur as a side effect of certain types of anticancer therapy, especially biological therapies, such as interferon or interleukin. Also called flu-like syndrome.

What is the cause of influenza? ›

The flu is caused by an influenza virus. Most people get the flu when they breathe in tiny airborne droplets from the coughs or sneezes of someone who has the flu. You can also catch the flu if you touch something with the virus on it, and then touch your mouth, nose, or eyes.

What is flu or influenza? ›

What is Influenza (Flu)? Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death.

What type of virus is influenza? ›

Influenza viruses are members of the family Orthomyxoviridae. This family represents enveloped viruses the genome of which consists of segmented negative-sense single-strand RNA segments.

How is influenza spread? ›

Person to Person. People with flu can spread it to others. Most experts think that flu viruses spread mainly by droplets made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby (usually within about 6 feet away) or possibly be inhaled into the lungs.


1. Know the common symptoms of flu
(Centers for Disease Control and Prevention (CDC))
2. How to Know if You Have the Flu: Flu Symptoms
(Centers for Disease Control and Prevention (CDC))
3. Influenza (Flu Virus): Symptoms, Causes, and Vaccines with Dr. Edward Ryan | Mass General Brigham
(Mass General Brigham)
4. Warning Signs: Seasonal Flu
(Centers for Disease Control and Prevention (CDC))
5. What is Seasonal Flu and Swine Flu? Causes, Symptoms and Treatment | Dr. Chandrashekar
(Narayana Health)
6. Flu Symptoms and Diagnosis


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