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Influenza Infection

Written by Ada’s Medical Knowledge Team

Updated on

What is influenza infection?

Influenza infection, also known as the flu, is a common viral infection of the airways that most often occurs in fall and winter. It can cause multiple symptoms, including fever, cough, a runny nose and muscle aches. 1

Influenza can be caused by a number of different viruses and is spread from person to person, primarily through sneezing and coughing. 2 When an infected person sneezes, coughs or talks, they may send infected water droplets into the air. It is then possible for a nearby person to inhale these and contract the infection themselves. It is also possible to catch influenza indirectly by touching a contaminated surface or object and then touching the eyes, nose or mouth. 3

Because influenza is so infectious, people with mild cases of the flu are recommended to avoid contact with others by staying at home during their illness. In most cases, no specific treatment is needed, and people recover from flu within a week. 4

However, people with severe cases of the infection, or who are at higher risk of developing flu-related complications, may need to seek medical attention. 5 In rare instances, influenza can cause severe complications, in which case the condition may become life-threatening.

Types of influenza infection

There are two main types of influenza infection that affect humans, known as influenza A and influenza B. These two viruses are responsible for the majority of seasonal flu epidemics that break out each year, with most cases occurring during the winter period. 6

Influenza A is believed to cause more than 70 percent of flu cases in a typical season. Of the remaining 30 percent of flu cases, the majority are caused by influenza B. 7 Both types of the virus result in similar flu symptoms.

The viruses that cause influenza A are constantly evolving, meaning that new strains of the infection develop fairly often. 5 The influenza B virus is also able to develop over time, although at a slower rate than influenza A.

Because the viruses are constantly changing, it is possible to contract influenza infection multiple times in a lifetime. Falling ill with one strain of the flu may help to build immunity against that particular strain, but it is unlikely to protect against a different strain in the future. 8 Annual flu shots are typically updated every year in order to vaccinate against the influenza strain, or strains, that is/are expected to be most common that year. 9

Good to know: There are two further types of influenza virus: influenza C and D. However, influenza type C occurs much less frequently in humans and only causes a mild respiratory illness. Influenza type D mostly affects cattle and is not known to infect humans at all. 10

Influenza infection symptoms

Influenza infection can cause mild to severe symptoms that are variable from person to person. Symptoms can come on very suddenly and most often begin to appear one to four days after infection. 4 However, in some cases the onset of influenza symptoms can occur a matter of hours after exposure. 11

Common symptoms of influenza infection

Typical signs and symptoms of influenza infection include: 1 5 7 12

Respiratory symptoms, such as coughing and a sore throat are often minimal at first, but can develop to become more prominent as the infection progresses. 9 Vomiting and diarrhea may also be present; this is more common in children than adults. 12

The severity of many symptoms can range from mild to severe. For example, while a fever is a common symptom of influenza infection, it might be a low fever of around 100 F / 37.7 C or a high fever of around 104 F / 40 C. Other people with influenza infection may simply feel feverish, without displaying a high temperature at all. 13

Symptoms usually peak within the first couple of days of their onset and then begin to subside. Coughing, weakness and tiredness are often the final symptoms to disappear. Most people with an influenza infection recover within a week. 9

Symptoms of severe influenza infection

While unpleasant, typical flu symptoms do not usually require medical intervention. 14 However, there are a number of additional symptoms which may indicate a more serious infection or the presence of a flu-related complication, such as pneumonia. Symptoms that may be a warning sign of severe influenza sickness include: ref13 15

  • Shortness of breath or fast breathing
  • Increased heart rate
  • Chest or abdominal pain
  • Recurrent high fever
  • Dizziness
  • Confusion

In children, additional symptoms may include:

  • Trouble breathing
  • Bluish tinge to the skin
  • Inability or unwillingness to eat or drink
  • Fever with a rash

Such symptoms may be an indication of a more serious influenza infection or of a flu-related complication. People who are exhibiting more severe influenza infection symptoms should seek medical attention.

Flu vs. cold

It can be difficult to tell the difference between influenza infection and the common cold. Both are respiratory illnesses that often cause similar symptoms, such as coughing, sneezing and a sore throat. However, identifiable differences to look out for include: 16

  • Flu symptoms are usually more intense than cold symptoms
  • Flu symptoms tend to arrive quickly, whereas cold symptoms appear more gradually
  • Fever and headaches are common symptoms of flu, but are rare with colds
  • Flu can cause more serious complications, such as pneumonia. It is not usual for a cold to result in any other health problems

If you are concerned you may have influenza infection or a cold, you can download the free Ada app to begin your personal health assessment.

Causes of influenza infection

Influenza infection is caused by an influenza virus. It is a highly contagious infection that can affect anyone. Because it is such a common condition, exact data on how many people contract the flu each year is not readily available. However, Centers for Disease Control and Prevention estimate that the flu has caused between 9.3 million to 49 million illnesses annually in the United States since 2010. 17

Influenza infection is usually contracted through the inhalation of contaminated water droplets. Often, these water droplets are released through the nose or mouth of an infected person. Three common ways in which the virus is spread are: 3 9

  • Airborne droplets. When an infected person coughs, sneezes or talks, they can release water droplets into the air. These airborne droplets may then be inhaled by another person
  • Direct contact. Direct contact with the water droplets of an infected person, e.g. sharing household utensils or kissing
  • Indirect contact. Handling an object or surface which has been contaminated with the influenza virus, e.g. touching a used tissue

Good to know: It is very unusual for humans to contract influenza infection direct from animals. When transmission does occur, it is predominantly the result of direct contact with infected animals, e.g. through farming, slaughter, food preparation, exposure to water contaminated by droppings. 18

Risk factors for influenza infection

Although anybody can contract influenza infection, the majority of people with the illness are able to recover at home without the need for hospitalization or medical care. Most affected people make a full recovery within a week. 4

However, there are certain groups of people who are at higher risk of developing flu-related complications. Such groups include: 5 7 1 19

  • Adults over the age of 65
  • Children under the age of five, in particular under the age of two
  • People living in closed environments such as nursing homes or other care facilities
  • Pregnant people and people up to two weeks after giving birth
  • Native Americans
  • People with certain pre-existing medical conditions

Medical conditions which may increase the risk of flu-related complications developing include: 1 5 7 19 20

Good to know: In the United States, the flu season is considered to take place during fall and winter, with incidences commonly peaking between December and February. 17 21 People located in the Northern Hemisphere are therefore at higher risk of contracting influenza infection during these months, although infection can occur at any time of the year. The Southern Hemisphere also typically experiences a winter flu season, which usually takes place between April and September. 22

Diagnosing influenza infection

Diagnosis of influenza infection is usually based on the symptoms experienced and an evaluation by a doctor. Diagnosis may be particularly swift if there is a known presence of a flu outbreak in the local community. Further tests are often not necessary because a clinical evaluation is usually enough to recommend a course of treatment. 23

However, further diagnostic tests may be recommended if:

  • Diagnosis is uncertain
  • Symptoms are severe
  • The affected person is at higher risk of developing complications
  • An outbreak of respiratory illness has occurred in the local community, and the test may be able to confirm flu as the cause

Two common tests used to help diagnose influenza infection are rapid influenza diagnostic tests (RIDTs) and rapid molecular assays. Both tests involve swabbing a person’s nose or throat and testing the mucus for the virus. Results can often be produced within 20 minutes. However, while the tests are quick and simple to perform, results are not as accurate as other diagnostic methods. It is possible to still receive a flu diagnosis, even with a negative test result. 23

Despite their limitations, the speed and simplicity of RIDTs can make them useful in promptly identifying influenza as the cause of a respiratory disease outbreak, particularly in care settings such as hospitals and nursing homes. If RIDTs confirm positive results in one or more people with suspected influenza, a swift decision can then be made to implement infection control measures and administer treatment.

Blood tests may be suggested if further complications are suspected. 5 If the person is thought to have developed pneumonia or is displaying significant respiratory problems, a chest X-ray may also be conducted. 9

Good to know: Because influenza infection is so common, it is often correctly suspected by the affected person before medical diagnosis takes place. 7 Many people therefore choose to stay at home and wait for the infection to pass of its own accord. However, if the affected person is worried about their symptoms, is very sick or is in a high risk group for developing complications, medical care should be sought. 14

Worried that you or a loved one may have influenza infection? Download the free Ada app to begin your personal health assessment.

Treatment of influenza infection

There is no specific medicine used to target the influenza virus itself. Instead, treatment aims to manage and ease the symptoms of influenza, until the body’s natural immune system is able to clear the infection. 7

At-home treatment

Most people are able to recover from influenza infection at home, without the need for medical intervention. 2 Home remedies that may help manage the symptoms of influenza include: 5 7 25

  • Staying hydrated by drinking lots of fluids, such as water
  • Resting and avoiding strenuous activities
  • Reducing fever and aches with acetaminophen, or nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen
  • Reducing cold-like symptoms, such as a runny nose and sore throat, with common cold treatment methods such as nasal decongestants and oral lozenges

Good to know: Although aspirin can be used to reduce fever and aches, it is not recommended for use by children or adolescents under the age of 18. This is because of the risk of a rare but serious condition called Reye’s syndrome, which can result in liver and brain damage. 26 Acetaminophen and ibuprofen are generally used as suitable alternatives.

Antiviral medication

People with a severe case of the flu, or those who are at greater risk of developing complications, may be prescribed antiviral medications by their doctor. 5 People who are at high risk of developing complications may also be prescribed antiviral medication as a preventive measure, in order to reduce the risk of contracting the flu altogether. 27

The most common antiviral medications used are: 7 28

  • Oseltamivir, typically swallowed as a pill or liquid
  • Zanamivir, typically inhaled as a powder
  • Peramivir, typically injected as a liquid

These medications target both influenza type A and type B. They do not kill the virus, but they can lessen the symptoms and shorten the period of illness by a couple of days. 29

Antiviral drugs are also used to reduce the risk of the affected person developing complications. These drugs are most effective when used within the first 48 hours of symptoms first appearing. 5

Good to know: Because influenza infection is viral and not bacterial, antibiotics are not effective in treating the illness. However, antibiotics may be prescribed if the affected person develops an additional bacterial infection as a complication of influenza. 7

Preventing influenza infection

The best precaution against influenza infection is to be vaccinated against it.

Flu vaccine

Centers for Disease Control and Prevention (CDC) recommends an annual flu vaccination for all people in the U.S. over the age of six months old. 30 It is considered the best way to help prevent seasonal flu and its potential complications. The flu vaccine is especially important for people who are at higher risk of developing flu-related complications, such as the elderly or people with certain medical conditions.

Vaccinations are updated on an annual basis, in order to protect against the most prevalent strains of the influenza virus that year. 9 It is recommended that people receive a flu vaccine in early fall, before the flu season begins. 17 The two main vaccination methods are: 7

  • Flu shot. An injection is the more common vaccination method and can be given to anyone aged six months or over, including pregnant people. People over the age of 65 may receive a slightly higher dose of the vaccine.
  • Flu nasal spray. This vaccination method is typically reserved for otherwise healthy people between 2 to 49 years old. It is not suitable for certain people, including pregnant people, those with a weakened immune system, children under the age of 18 who take long-term aspirin medication or asthmatic children aged between two to four years old.

Good to know: Most people are able to receive one of the above flu vaccination methods. However, there are multiple vaccination options, and talking to a doctor can be useful to help people decide which is the best option for them each year.

Flu vaccine benefits

Vaccination against the flu has multiple benefits, including:

  • Preventing influenza infection. Centers for Disease Control and Prevention estimates that flu vaccination prevented around 5.3 million influenza illnesses in the United States during 2016-2017 31
  • Preventing influenza-related hospitalization. A 2017 study found that the vaccination in the United States reduced the number of adult flu-related hospitalizations by an average of around 40 percent 32
  • Reducing the severity of influenza infection. In the United States, flu vaccination has resulted in reduced flu-related deaths, intensive care admissions and hospitalization duration, according to one 2017 study 33
  • Life-saving in children. A Centers for Disease Control and Prevention study in 2017 was the first of its kind to show that a flu vaccination can significantly reduce the a child’s risk of dying from influenza infection 34

Good to know: In addition to the above benefits, receiving the flu vaccine also helps to protect other people in the community, including those who are at greater risk of developing serious flu-related illness. 31


Due to how contagious influenza viruses are, it is important to limit the risk of spreading the infection to others. For this reason, people with the flu are recommended to stay at home until around five to seven days after first becoming ill. Children and people with a weakened immune system can take longer than seven days to recover from the illness and therefore may remain infectious for longer. 3

As flu symptoms often only appear a couple of days after becoming infected, it is possible for a person to pass on the flu virus before realizing they themselves are infected. It is also possible to contract the flu but display no symptoms, however these people are still able to spread the infection to others. 5


Practicing good personal hygiene can also help prevent the spread of influenza infection. Useful preventative measures include: 14

  • Washing hands frequently, ideally with soap and water, or hand sanitizer
  • Covering the mouth and nose with a tissue when coughing or sneezing
  • Avoiding close contact with people who are sick
  • Avoiding contact with the eyes, nose or mouth unless hands are fully clean

Preventive antiviral medication

As well as being used to treat influenza infection, antiviral medication may occasionally be prescribed as a preventive measure to reduce the likelihood of a high-risk person contracting the condition and developing influenza-related complications. 27 This preventive approach is known as prophylaxis.

Because the flu vaccine is considered to be the best precaution against influenza infection, antiviral prophylaxis is usually considered only in specific circumstances. Typically, it is only suggested to high-risk people who have been exposed to another person with influenza, and also fall into certain categories, such as: 27

  • They received the flu vaccine within the last two weeks
  • They cannot receive the flu vaccine, e.g. due to an allergy
  • They may not be responsive to the flu vaccine, e.g. people receiving immunosuppressive medications

Antiviral prophylaxis may also be used to control an influenza outbreak among high-risk people in long-term care facilities, such as nursing homes. In such cases, antiviral medication may be prescribed to both residents and staff. 27

Complications of influenza infection

While the majority of people who contract influenza infection make a full recovery, complications can occur sometimes. Complications are more likely to develop in certain groups of people, as outlined in the risk factors section above.

Some complications can develop to become severe or even life-threatening. Pneumonia is a particularly common and serious potential complication of the flu. 12 At-risk groups of people, such as the elderly, or those with certain pre-existing medical conditions, should contact a doctor if flu is suspected in order to limit the risk of developing a flu-related complication.

Moderate influenza complications

Moderate complications that can arise from influenza infection include: 5 12 15

Moderate complications are generally not thought to cause any long-term health problems, although some may require additional medical treatment.

Severe influenza complications

There are a number of severe complications that can develop through having influenza. Pneumonia is the most common complication of influenza infection, which can develop to become a potentially life-threatening condition. 7

Pneumonia and influenza

Pneumonia is a serious infection of the lungs and a common complication of influenza. Flu-related pneumonia can either develop as a direct result of the influenza virus, known as primary viral pneumonia, or, more commonly, due to a combination of the virus and an added bacterial infection, such as bacterial bronchitis. This is known as secondary bacterial pneumonia. 5

Symptoms that may indicate the development of either primary viral or secondary bacterial pneumonia, include: 7 15 16

  • Shortness of breath
  • Fast breathing
  • Increased heart rate
  • Chest pain
  • Recurrent high fever
  • A deep, wet cough that produces mucus or phlegm, rather than the dry cough that is a common symptom of influenza
  • Bloody phlegm

Good to know: Around 90 percent of acute bronchitis cases in otherwise healthy adults are caused by viruses, such as influenza. 35

Other severe complications

Other potentially severe complications of influenza infection include: 5 9 12 36

  • An inflammation of the muscles, known as myositis
  • An inflammation of the heart muscle, known as myocarditis
  • An inflammation of the brain, known as encephalitis
  • Sepsis
  • Worsening of certain existing medical conditions, such as asthma or chronic heart failure

Good to know: Medical help should be sought by anyone who is worried they may have developed a complication due to influenza infection, whether moderate or severe. You can also check your symptoms with the free Ada app.

Influenza infection FAQs

Q: Is influenza infection contagious?
A: Yes, influenza infection is very contagious. It is believed that people infected with the flu virus can spread it to other people up to six feet away. 3 The infection most commonly spreads through airborne water droplets, released as a result of sneezing or coughing. It is also possible to catch the infection by coming into direct contact with someone who has influenza infection, as well as by touching a contaminated object or surface.

Q: Does flu give you a rash?
A: It is not typical to flu to result in a skin rash. However, there are a small number of reported cases where a rash has accompanied the flu, particularly in children. 37 38

If a skin rash develops during influenza infection, it is recommended to contact a doctor in order to rule out any other possible causes of the rash. Measles is another contagious virus that can cause respiratory symptoms. However, unlike the flu, it usually also causes a skin rash. This means, it is possible to mistake the early symptoms of measles for the flu. 39

Q: What home remedies can be used to treat the flu?
A: Treatment of the flu focuses on alleviating the symptoms of the illness. Many people are able to recover from influenza infection at home, without needing to visit a hospital or receive medical intervention. Remedies that can be used at home to help ease the signs of flu include:

  • Staying hydrated by drinking plenty of water
  • Resting
  • Reducing fever and aches with acetaminophen, or nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen
  • Reducing cold-like symptoms with common cold treatment methods

Q: Can flu be treated with antibiotics?
A: No, influenza viruses cannot be treated with antibiotics. Because the infection is viral and not bacterial, antibiotics are not effective. If medicine is required to treat the flu, antiviral medications are usually prescribed. However, antibiotics may be prescribed if the affected person also develops a secondary bacterial infection as a result of the flu, such as a chest infection.

Q: Is there a flu vaccine?
A: Yes, an annual flu vaccine is recommended for all people over this age of six months old. This vaccination, which is usually administered as an injection, protects against the most common strains of influenza each year. It is considered the best way to help prevent seasonal flu. In certain cases, the vaccine may be administered as a nasal spray rather than a flu shot.

As well as helping to prevent influenza infection, the annual flu vaccine has also been shown to reduce the severity of the infection if it is still caught. Studies show that this has since resulted in a reduction in the number of influenza-related hospitalizations and deaths each year in the United States.

Q: How else can the flu be prevented?
A: The flu vaccine is considered the best way to help prevent seasonal flu and its potential complications. In addition to this, affected people are recommended to stay at home until around five to seven days after first becoming ill to avoid spreading the infection. Good hygiene measures, such as regularly washing hands and sneezing or coughing into a tissue, can also help to prevent the spread of influenza.

Q: What is the difference between influenza A and influenza B?
A: Although influenza A and B both cause similar symptoms and can be treated in the same manner, there are a number of differences between the viruses.

Influenza A can affect both humans and animals and is the most common type of influenza infection. Influenza B is less common, but the infection and symptoms experienced can be just as severe. 40 Unlike influenza A, influenza B is generally thought not to infect animals and can only be spread from human to human. Both viruses are able to evolve over time, resulting in different strains. 10

Q: Can influenza infection cause an epidemic?
A: Yes. An epidemic is a sudden increase in cases of a disease, above what is usually expected, within a specific geographic area or community. 41 Influenza infection causes seasonal epidemics every year in the United States, generally during the fall or winter months and commonly peaks between December and February. 17 Influenza infection can also cause a pandemic, which is an epidemic that has spread across various countries or even continents. However, influenza pandemics are rare.

Q: What is seasonal flu?
A: Seasonal flu is another term for an influenza infection that occurs during the typical flu season in the fall and winter months. Because the virus spreads most commonly during these months, it is recommended that people in the Northern Hemisphere receive a flu vaccine in early fall, before the season begins. 17

Other names for the influenza infection

  • Flu
  • Seasonal flu
  • Grippe

[46]: Centers for Disease Control and Prevention. “Rapid Influenza Diagnostic Tests.” 25 October 2016. Accessed 1 February 2019.