Influenza (Flu)

ByBrenda L. Tesini, MD, University of Rochester School of Medicine and Dentistry
Reviewed/Revised Apr 2022 | Modified Sep 2022

Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general feeling of illness (malaise).

  • The virus is spread by inhaling droplets coughed or sneezed out by an infected person or by having direct contact with an infected person's nasal secretions.

  • Influenza often starts with chills, followed by a fever, muscle aches, headache, a sore throat, a cough, a runny nose, and a general feeling of illness.

  • Influenza is often diagnosed based on symptoms.

  • An annual influenza vaccination is the best way to prevent influenza.

  • Resting, drinking plenty of fluids, and avoiding exertion can help people recover, as can taking pain relievers, decongestants, and sometimes antiviral drugs.

Influenza is distinctly different from the common cold. It is caused by a different virus and produces symptoms that are more severe. Also, influenza affects cells much deeper down in the respiratory tract.

Transmission of influenza

The influenza virus is spread by

  • Inhaling droplets that have been coughed or sneezed out by an infected person

  • Having direct contact with an infected person's nasal secretions

  • Handling household articles that have been in contact with an infected person or an infected person's secretions

Influenza types and strains

There are three types of influenza virus:

  • Type A

  • Type B

  • Type C

There are many different strains within types A and B influenza viruses but all cause similar illness. Different strains cause the regular seasonal outbreaks of flu. Type C does not cause a typical influenza illness.

Type A causes most influenza cases (usually more than 70% in a typical season), and most of the others are caused by type B. Type C influenza occurs less frequently, mainly in children.

The strain of influenza virus causing outbreaks is always changing slightly, so that each year the influenza virus is a little different from the previous year's. It often changes enough that previously effective vaccines no longer work.

Influenza type A strains are named based on the specific versions of two proteins present on the surface of the virus. The proteins are H (for hemagglutinin) and N (for neuraminidase). There are 18 different H proteins and 11 N proteins. Thus, a strain might be named influenza A, H1N1. An H1N1 strain was responsible for the so-called swine flu pandemic in 2009–2010. (A pandemic is a major worldwide epidemic.) More recently, H3N2 strains have been causing most infections in the United States.

The name of a strain often reflects its type, the location where it first appeared (for example, Hong Kong flu) or an animal (for example, swine flu), and the year it was detected.

Influenza epidemics and pandemics

In an influenza epidemic, many people get sick within a very brief period of time. Every year, throughout the world, widespread outbreaks of influenza occur during late fall or early winter in temperate climates (called seasonal epidemics). Influenza epidemics may occur in two waves:

  • First, in schoolchildren and the people who live with them

  • Secondly, in people who are confined to home or who live in long-term care facilities, mainly older people

In each epidemic, usually only one strain of influenza virus is responsible for the disease.

An influenza pandemic refers to an outbreak that has spread across a large region, typically across continents and sometimes even worldwide. There have been only 6 major influenza pandemics since 1889. An influenza pandemic is concerning because it usually occurs only when there has been a larger than usual change in the strain of influenza virus. When such a large change occurs, that strain of influenza may affect many more people and cause more severe illness. The risk of death is higher. Although no one knows for certain, scientists think the 1918 influenza pandemic caused 30 to 50 million deaths worldwide, including about 675,000 in the United States.

In 2009–2010, there was an epidemic of the H1N1 strain of influenza virus (see Influenza types and strains) that became widespread and was considered a pandemic. This strain had a combination of genes from pig (swine), bird, and human influenza viruses. Because the first reports focused on the swine component, it was referred to publicly as "swine flu," although it was not acquired directly from pigs. People acquired this influenza virus infection from other infected people (person to person spread) like ordinary flu.

Symptoms of Influenza

Symptoms of influenza start 1 to 4 days after infection and can begin suddenly. Chills or a chilly sensation is often the first indication. Fever is common during the first few days, sometimes reaching 102 to 103° F (about 39° C). Many people feel so ill, weak, and tired that they remain in bed for days. They have aches and pains throughout the body, particularly in the back and legs. Headache is often severe, with aching around and behind the eyes. Bright light may make the headache worse.

At first, respiratory symptoms may be relatively mild. They may include a scratchy sore throat, a burning sensation in the chest, a dry cough, and a runny nose. Later, the cough can become severe and bring up phlegm (sputum).

The skin may be warm and flushed, especially on the face. The mouth and throat may redden, the eyes may water, and the whites of the eyes may become bloodshot. People, especially children, may have nausea and vomiting. A few people lose their sense of smell for a few days or weeks. Rarely, the loss is permanent.

Most symptoms subside after 2 or 3 days. However, fever sometimes lasts up to 5 days. Cough, weakness, sweating, and fatigue may persist for several days or occasionally weeks. Mild airway irritation, which can result in a decrease in how long or hard a person can exercise, or slight wheezing may take 6 to 8 weeks to completely resolve.


The most common complication of influenza is

In viral pneumonia, the influenza virus itself spreads into the lungs. Some people with viral pneumonia also become infected with bacteria and develop bacterial pneumonia; this can happen when bacteria (such as pneumococci or staphylococci) attack the person's weakened defenses. With either, people may have a worsened cough, difficulty breathing, persistent or recurring fever, and sometimes blood or pus in the sputum.

People at high risk of complications and death from influenza include

  • Children under age 5 years; children under age 2 years are at particularly high risk

  • Adults over age 65 years

  • People with chronic medical disorders (particularly those that affect the heart, lungs, kidneys, liver, or immune system) or diabetes mellitus

  • People with extreme obesity (body mass index [BMI] of 40 or more)

  • Women in the 2nd or 3rd trimester of pregnancy and up to 2 weeks after delivery

  • People with disorders that increase their risk of choking on oral secretions, such as stroke or other neurologic disorders that cause weakness, and seizure disorders

Diagnosis of Influenza

  • A doctor's evaluation

  • Sometimes tests on samples of blood or respiratory secretions

  • Sometimes a chest x-ray and measurement of oxygen levels in the blood

Because most people are familiar with the symptoms of influenza and because influenza occurs in epidemics, it is often correctly diagnosed by the person who has it or by family members. The severity of symptoms and the presence of a high fever and body aches help distinguish influenza from a common cold, especially when the illness occurs during an influenza outbreak. It is more difficult to correctly identify influenza by symptoms alone when no outbreak is occurring.

Tests on samples of respiratory secretions can be used to identify the influenza virus. Blood tests may help determine how sick a person is with the infection. Such tests are done mainly when people appear very ill or when a doctor suspects another cause for the symptoms. Some tests can be done in the doctor's office.

If doctors suspect that pneumonia has developed, they take a chest x-ray and measure oxygen levels in the blood with a sensor placed on a finger (pulse oximetry).

Prevention of Influenza

Prevention consists of

  • Vaccination each year for everyone 6 months of age or older (with rare exceptions)

  • Sometimes antiviral drugs

Prevention is important for all people, but particularly for health care workers and those who are at high risk of complications of influenza.

Did You Know...

  • All people 6 months old and over, with rare exceptions, should get a flu vaccination each year.

Vaccines for influenza

Annual vaccination is the best way to avoid getting influenza.

There are two basic types of influenza vaccine:

  • A vaccine that is given by injection and that contains inactivated (killed) influenza virus or pieces of the virus

  • A vaccine that is inhaled as a nasal spray and that contains weakened (attenuated) live viruses

The injected, inactivated vaccine can be given to all people aged 6 months old or older, including pregnant women. A higher-dose inactivated influenza vaccine is available for people over age 65. The inactivated influenza vaccine is not given to

  • People who had a severe allergic reaction to any influenza vaccine or one of its components

The inhaled, live-virus vaccine is used only in healthy people aged 2 to 49 years. It is not given to

  • People who had a severe allergic reaction to any influenza vaccine or one of its components

  • Pregnant women

  • People with a weakened immune system (such as those who have AIDS or who take drugs that suppress the immune system) and, in some cases, those who live with them

  • Children who are aged 2 to 4 years and have asthma or have had wheezing or asthma attacks in the last 12 months

  • People who have taken an antiviral drug for influenza in the past 2 days

The inhaled, live-virus vaccine was not recommended for the 2016-2017 and 2017-2018 flu seasons because of concerns that it was not as effective as the inactivated vaccines. However, the inhaled, live-virus vaccine has been reformulated and is now recommended equally with the inactivated vaccines. Some experts recommend the inactivated vaccine be used for children until the new live-virus vaccine has been studied more.

Doctors typically postpone giving either vaccine to people who are ill at the time of vaccination.

After the 1976 swine flu epidemic, in which millions of people received the influenza vaccine, a larger than usual number of people had a nerve disorder called Guillain-Barré syndrome. At the time, doctors thought the vaccine triggered Guillain-Barré syndrome, but more recent evidence makes this connection uncertain. However, to be safe, doctors use caution when giving another influenza vaccination to anyone who developed Guillain-Barré syndrome within 6 weeks of receiving the influenza vaccine. In such cases, doctors and the person weigh the risk of the person's getting influenza against the risk of possibly developing Guillain-Barré syndrome.

People with a severe allergy to eggs may have an allergic reaction to the influenza vaccine because the vaccine is made from viruses grown in eggs. So doctors follow the following guidelines when giving people with an egg allergy the influenza vaccine:

  • If the only reaction people have had to eggs is a rash (hives), they can get the influenza vaccine.

  • If people have had a severe allergic reaction (such as anaphylaxis) after they are given the influenza vaccine, they are not given the influenza vaccine again.

Also, two vaccines that do not contain egg are available: one that can be used in people aged 18 and older and one that can be used in people aged 4 years and older.

Influenza vaccines usually protect against three or four different strains of influenza virus. The strains of virus that cause influenza outbreaks change each year. Thus, different vaccines are developed each year to keep up with changes in the virus. Experts try to predict the strain of virus that will attack each year based on the strain of virus that predominated during the previous influenza season and the strain causing disease in other parts of the world. When the H and N proteins in the vaccine match those in the influenza strains causing the current epidemic, the vaccine decreases the rate of infection by 70 to 90% in healthy adults.

In older people who live in long-term care facilities, the vaccine is less likely to prevent influenza, but it reduces the chances of developing pneumonia and of dying. Because the immune system becomes weaker as people age, a high-dose influenza vaccine has been designed specifically for people 65 years and older. This high-dose vaccine can stimulate a stronger immune response in older people.

Other than occasional soreness at the injection site or, for the inhaled vaccine, a runny nose, side effects from the vaccine are rare.

In the United States, vaccination takes place during the fall so that levels of antibodies are highest during the peak influenza months: November through March. For most people, about 2 weeks is needed for the vaccination to provide protection. Children aged 6 months to 8 years being given the influenza (flu) vaccine for the first time should be given two doses of the vaccine at least 4 weeks apart.

Influenza vaccine can be given at the same time as COVID-19 vaccine.

Antiviral drugs

Although vaccination is the preferred method of prevention, several antiviral drugs can be used in certain people to prevent infection with the influenza virus.

During an outbreak of influenza, antiviral drugs are given to people who have been vaccinated within the previous 2 weeks (because it takes 2 weeks for the vaccine to become effective). They are stopped 2 weeks after people are vaccinated. These drugs are also given to people who have conditions that make vaccination ineffective or dangerous.

Treatment of Influenza

  • Rest and plenty of fluids

The main treatment for influenza is to rest adequately, drink plenty of fluids, and avoid exertion. Normal activities may resume 24 to 48 hours after the body temperature returns to normal, but most people take several more days to recover.

Reye syndromeOther measures as listed for the common cold, such as nasal decongestants and steam inhalation, may help relieve symptoms.

The same antiviral drugs

If a bacterial infection develops, antibiotics are added.

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