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Influenza

Last updated June 1, 2018

Approved by: Maulik P. Purohit MD MPH

National Institute of General Medical Sciences

Influenza, or more commonly ‘The Flu’, is an infection caused by a virus that affects the respiratory system. This illustration shows an influenza virus attaching to host membrane (with labels).


What are the other Names for this Condition? (Also known as/Synonyms)

  • Grippe
  • Seasonal Influenza
  • The Flu

What is Influenza? (Definition/Background Information)

  • Influenza, or more commonly ‘The Flu’, is an infection caused by a virus that affects the respiratory system. There are 2 major types of Influenza viruses, known as the Influenza A virus and Influenza B virus, which cause ‘The Flu’
  • It must be mentioned that both type A and type B viruses are highly mutable and give rise to a number of strains or subtypes. The subtypes could change each flu season. Additionally, the Influenza virus can get transmitted from animals, such as pigs or birds, to humans, adding to the various existing strains of the virus types
  • Influenza is a common condition that occurs worldwide. Although it has the potential to affect any population, young children, the elderly, and those with certain pre-existing conditions or immunosuppression are considered most susceptible to this disease
  • Since the Influenza virus spreads easily through air, via infected respiratory droplets (cough and sneeze), those living in close proximity to others (such as in the military barracks, nursing homes, and other crowded spaces) are also considered at risk for contracting ‘The Flu’
  • The common signs and symptoms of Influenza include fever, body ache, headache, sore throat, cough (non-productive), weakness, and lethargy
  • Generally, no treatment is necessary for Influenza; addressing the symptoms, taking adequate rest, and drinking plenty of fluids are helpful. If an individual is affected, he/she should stay indoors, in order to avoid infecting others
  • Commonly, healthy individuals with flu recover in about a week’s time. However, the very young, the elderly, immune-compromised individuals and those with serious pre-existing conditions can develop complications (such as pneumonia), and hence, may take longer to recover
  • Getting vaccinated for Influenza every year, frequent hand washing, not sharing items with infected individuals, cleaning surfaces touched by multiple people frequently, are factors that can help contribute towards preventing ‘The Flu’ from spreading

Each flu season is generally different, with the severity of symptoms and spread of Influenza dictated by the subtypes of viruses, the efficiency of vaccines, the number of people that get vaccinated that season, the availability of vaccines, and several other factors.

Who gets Influenza? (Age and Sex Distribution)

  • Influenza is prevalent around the globe, usually affecting all nations. People belonging to all racial groups and ethnicities can be affected
  • According to the World Health Organization, between 5-15% of the global population is affected by The Flu every year, and the disease is responsible for approximately 200,000-500,000 deaths annually
  • The Influenza virus affects individuals of both genders and all ages. However, young children, the elderly, those with chronic pre-existing conditions, and immune-compromised individuals, are considered especially at risk

What are the Risk Factors for Influenza? (Predisposing Factors)

The most common risk factors that could predispose an individual to being infected with the Influenza virus are:

  • Age: Being very young or elderly
  • Pre-existing conditions or chronic conditions such as diabetes, lung and heart conditions, etc.
  • Pregnancy: Women who are pregnant or have just delivered a baby are reported to be more prone to being infected with the flu virus, owing to changes in their immune system, hormones, heart and lung systems, during pregnancy
  • Weak immune system: HIV/AIDS patients, organ transplant patients on drugs to suppress their bodies’ natural immunity all have a higher risk
  • Healthcare workers and daycare workers are more at risk for exposure to the virus
  • Living in close proximity to other people, or in crowded spaces such as nursing homes, military barracks, and refugee camps

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases ones chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Influenza? (Etiology)

Influenza is a viral infection caused by the Influenza virus. It is classified into 2 major types - the Influenza A and B virus.

  • Numerous sub-types exist for each type of Influenza virus. The sub-types come into being as a result of adaptation of the virus
  • Additionally, the viruses can cross-infect humans from animals, such as pigs or birds, too. This is often the cause of worldwide epidemics
  • The virus particles get transmitted through airborne respiratory droplets (such as from a sneeze or cough), as well as when a individual touches a surface touched by an infected individual (such as an elevator button, telephone, mouse, etc.), and unknowingly touches their face

What are the Signs and Symptoms of Influenza?

The symptoms of Influenza may be similar to the symptoms of a common cold. However, an important difference is that the onset of Influenza symptoms is often sudden, whereas a cold usually develops gradually.

The signs and symptoms of Influenza may include the following:

  • Fever and chills
  • Body aches, headaches
  • Sore throat
  • Cough (typically non-productive)
  • Tiredness
  • Rhinitis (allergy-like symptoms, such as runny or stuffy nose)
  • In some cases, vomiting and diarrhea

How is Influenza Diagnosed?

A healthcare provider might diagnose Influenza based on the following steps:

  • A physical examination, evaluation of symptoms (including ascertaining if the flu occurs during typical flu season)
  • Complete medical history including history of any medical conditions, immunosuppression
  • Complete blood count
  • Blood or sputum culture
  • Antigen detection test (for detecting viral particles, usually performed by taking a swab of nose or throat and getting it tested)
  • Chest X-ray, if pneumonia is suspected
  • Other tests as deemed appropriate, if complications arise

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Influenza?

Complications from Influenza usually occur in individuals who were in poor health before infection with the flu virus.

  • Such individuals might have ongoing, chronic health issues such as diabetes, lung or heart problems.
  • Additionally, if one’s immunity is weak, as in those with HIV/AIDS, or those under medication for suppressing their immunity (such as organ transplant patients), complications might develop
  • It has also been reported that obese individuals, pregnant women, and women who have just delivered a child, are more prone to complications from The Flu

Some potential complications of Influenza could include:

  • Spread of infection from the upper respiratory tract to the lungs, causing pneumonia 
  • Exacerbation of existing medical conditions
  • Bronchitis
  • Ear infections
  • Sinusitis
  • Myositis (an inflammation of the muscles)
  • Myocarditis (inflammation in the heart muscles), pericarditis (swelling of the membrane enclosing the heart)
  • Heart attack
  • Problems with the central nervous system
  • Pre-mature labor/delivery in pregnant women

How is Influenza Treated?

There are no treatment measures available for “curing” Influenza. However, the following might help provide some relief from symptoms in those affected by ‘The Flu’, along with a consultation of the healthcare provider:

  • Medications, such as oseltamivir, peramivir, and zanamivir, within 48 hours of start of symptoms (might help ease some of the symptoms)
  • Over-the-counter medications for reducing fever (adhering to the dosage instructions on the container is important)

Note: One should avoid giving aspirin to children; instead, acetaminophen, ibuprofen, or naproxen may be given, as recommended by a healthcare provider.

  • Over-the-counter painkillers for body pain and headache (following the dosage instructions on the container is important)
  • Use of nasal decongestants such as anti-histamines

Note: A patient of high blood pressure should consult with a healthcare professional before taking any decongestant, as pseudoephedrine and phenylephrine, commonly found in decongestants are known to increase the blood pressure.

  • Nasal spray for decongestion: Saline sprays are generally recommended and preferred over medicated sprays, which could have undesirable side effects. Consulting a healthcare provider is beneficial prior to usage
  • Cough syrup; lozenges (caution should be taken to not consume too many of these)
  • Drinking plenty of fluids
  • Rest
  • If secondary bacterial infections occur, then antibiotics may be prescribed

Note: Viral infections do not respond to antibiotics. Using antibiotics unnecessarily can lead to the evolution of antibiotic-resistant bacteria in one’s body.

How can Influenza be Prevented?

The following steps may be considered towards preventing or avoiding Influenza:

  • Annual flu vaccine is the best known method for preventing Influenza. It is generally a combination vaccine against 3 or 4 Influenza virus sub-types believed to be prevalent during that season. The sub-types could change every year, and hence the need for getting vaccinated every year
    • It is recommended that all individuals aged 6 months and over get the annual flu vaccine
    • The flu vaccine may be given as an injection or a nasal spray. The mode of administering live virus vaccines may be decided by the attending healthcare professional, depending on the individual’s age, health condition, pregnancy status in women, and other factors
    • The flu vaccine may only be partially effective, if additional subtypes of flu virus become prevalent during that particular season. Therefore, even with the vaccination, there is a likelihood of being infected by Influenza virus and having symptoms
  • During flu season, avoid crowded areas
  • Frequent hand washing (or the use of a hand sanitizer) goes a long way in preventing the spread of Influenza
  • When one shakes hands with people or uses common equipment or touches common surfaces (such as door knobs or staircase handrails), washing hands before touching one’s own face can help avoid an infection
  • If an individual is already infected, he/she should stay at home in order to avoid spreading the infection. The virus can spread when an individual is infected and is yet to show symptoms, as well as when symptoms are present
  • Contain coughs and sneezes: Cover mouth and nose when you sneeze or cough. To avoid contaminating your hands, cough or sneeze into a tissue or into the inner crook of your elbow. This will further reduce the spread of flu
  • Clean surfaces touched by multiple people frequently; such surfaces may include computer keyboards, elevator buttons, handrails, door handles, telephone receivers, etc.

What is the Prognosis of Influenza? (Outcomes/Resolutions)

  • The prognosis for Influenza is considered to be good in healthy individuals, who make a complete recovery within several days of flu symptoms onset
  • However, recovery from the flu might take longer in individuals who have chronic pre-existing conditions, particularly, if complications such as pneumonia develop

Additional and Relevant Useful Information for Influenza:

In the United States, 5-20% of the population is estimated to be affected by the flu every year, and over 200,000 people are hospitalized annually with flu-related complications.

What are some Useful Resources for Additional Information?


References and Information Sources used for the Article:


Helpful Peer-Reviewed Medical Articles:


Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: March 18, 2016
Last updated: June 1, 2018