The flu is an infection of the nose, throat, and lungs. It spreads easily.
This article discusses influenza types A and B. Another type of the flu is the swine flu (H1N1).
Influenza A; Influenza B; Oseltamivir (Tamiflu) - flu; Zanamivir (Relenza) - flu; Vaccine - flu
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.
People often confuse colds and flu. They are different, but you might have some of the same symptoms. Most people get a cold several times a year. On the contrary, people get the flu only once every few years.
Sometimes, you can get a virus that makes you throw up or have diarrhea. Some people call this the "stomach flu." This is a misleading name because this virus is not the actual flu. The flu mostly affects your nose, throat, and lungs.
Flu symptoms will often start quickly. You can start to feel sick about 1 to 7 days after you come in contact with the virus. Most of the time symptoms appear within 2 to 3 days.
The flu spreads easily. It can affect a large group of people in a very short amount of time. For example, students and co-workers get sick within 2 or 3 weeks of the flu's arrival in a school or workplace.
The first symptom is a fever between 102°F (39°C) and 106°F (41°C). An adult often has a lower fever than a child.
Other common symptoms include:
- Body aches
- Flushed face
- Lack of energy
- Nausea and vomiting
The fever, aches, and pains begin to go away on days 2 through 4. But new symptoms occur, including:
- Dry cough
- Increased symptoms that affect breathing
- Runny nose (clear and watery)
- Sore throat
Most symptoms go away in 4 to 7 days. The cough and tired feeling may last for weeks. Sometimes, the fever comes back.
Some people may not feel like eating.
The flu can make asthma, breathing problems, and other long-term (chronic) illnesses and conditions worse.
Exams and Tests
Most people do not need to see a health care provider when they have flu symptoms. This is because most people are not at risk for a severe case of the flu.
If you are very sick with the flu, you may want to see your provider. People who are at high risk for flu complications may also want to see a provider if they get the flu.
When many people in an area have flu, a provider can make a diagnosis after hearing about your symptoms. No further testing is needed.
There is a test to detect the flu. It is done by swabbing the nose or throat. Most of the time, test results are available very fast. The test can help your provider prescribe the best treatment.
Acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) help lower fever. Providers sometimes suggest that you use both types of medicine. DO NOT use aspirin.
A fever does not need to come all the way down to normal. Most people feel better when the temperature drops by 1 degree.
Over-the-counter cold medicines may make some of your symptoms better. Cough drops or throat sprays will help with your sore throat.
You will need a lot of rest. Drink plenty of liquids. 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 medicines.
Providers may give antiviral drugs to people who get very sick with the flu. You may need these medicines if you are more likely to have flu complications The health problems below may increase your risk of getting sicker with the flu:
- Lung disease (including asthma)
- Heart conditions (except high blood pressure)
- Kidney, liver, nerve, and muscle conditions
- Blood disorders (including sickle cell disease)
- A weakened immune system due to diseases (such as AIDS), radiation therapy, or certain medicines, including chemotherapy and corticosteroids
- Other long-term medical problem
These medicines may shorten the time you have symptoms by about 1 day. They work better if you start taking them within 2 days of your first symptoms.
Children at risk for a severe case of the flu may also need these medicines.
Millions of people in the United States get the flu each year. Most people get better within 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 of problems from the flu.
Anyone at any age can have serious complications from the flu. Those at highest risk include:
- People over the age of 65
- Children younger than 2 years old
- Women who are more than 3 months pregnant
- Anyone living in a long-term care facility
- Anyone with chronic heart, lung, or kidney conditions, diabetes, or a weakened immune system
Complications may include:
When to Contact a Medical Professional
Call your provider if you get the flu and think you are at risk for having complications.
Also, call your provider if your flu symptoms are very bad and self-treatment is not working.
You can take steps to avoid catching or spreading the flu. The best step is to get a flu vaccine.
If you have the flu:
- Stay in your apartment, dorm room, or home for at least 24 hours after your fever has gone.
- Wear a mask if you leave your room.
- Avoid sharing food, utensils, cups, or bottles.
- Use hand sanitizer often during 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 should receive the influenza vaccine. Children 6 months through 8 years of age may need 2 doses during a single flu season. Everyone else needs only 1 dose each flu season. For the 2019-2020 season, the CDC recommends use of the flu shot (inactivated influenza vaccine or IIV) and the recombinant influenza vaccine (RIV). The nasal spray flu vaccine (live attenuated influenza vaccine, or LAIV) may be given to healthy, non-pregnant people 2 through 49 years of age.
Aoki FY. Antiviral drugs for influenza and other respiratory virus infections. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 45.
Centers for Disease Control and Prevention website. Inactivated influenza VIS. www.cdc.gov/vaccines/hcp/vis/vis-statements/flu.html. Updated August 15, 2019. Accessed September 19, 2019.
Centers for Disease Control and Prevention website. Live, intranasal influenza VIS. www.cdc.gov/vaccines/hcp/vis/vis-statements/flulive.html. Updated August 15, 2019. Accessed September 19, 2019.
Centers for Disease Control and Prevention website. What you should know about flu antiviral drugs. www.cdc.gov/flu/antivirals/whatyoushould.htm. Updated April 22, 2019. Accessed September 19, 2019.
Havers FP, Campbell AJP. Influenza viruses. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 285.
Ison MG, Hayden FG. Influenza. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 340.
Treanor JJ. Influenza viruses, including avian influenza and swine influenza. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 165.
Reviewed By: Linda J. Vorvick, MD, Clinical Associate Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Internal review and update on 01/17/2020 by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.