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Last updated: January 6, 2022

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Influenza is a highly contagious disease that typically occurs during the winter months. It is caused by the influenza viruses A, B, and C. There are various subtypes of the type A viruses, which are classified based on their surface antigens H (hemagglutinin) and N (neuraminidase). Since the viruses constantly undergo genetic changes, new subtypes and strains develop fairly often. Most infected individuals do not show symptoms. However, if they are symptomatic, they typically present with sudden onset of high fever, headache, muscle/joint aches, nonproductive cough, and severe malaise. Inflammatory markers are usually normal or slightly elevated. Diagnosis can be established with the help of rapid antigen testing if acute infection is suspected (e.g., for hospitalized patients or nursing home residents). In some cases, bacterial superinfection (most commonly with Staphylococcus aureus and Streptococcus pneumoniae) may develop, which is associated with productive cough and high levels of inflammatory markers. Usually, supportive treatment (i.e., rehydration and antipyretics) is sufficient. However, antiviral therapy may be considered for patients with early or severe disease, especially those at high risk for complications. The most commonly used agents are the neuraminidase inhibitors oseltamivir and zanamivir. If therapy is initiated within 48 hours after the onset of symptoms, a decrease in symptoms, a shortened duration of disease, and a reduction of complications may be achieved. Hygiene precautions and vaccination (recommended for all persons aged 6 months and older) help prevent the spread of influenza viruses.

  • Distribution: worldwide
  • Seasonal pattern: Most infections occur during the fall and winter (influenza season).


Epidemiological data refers to the US, unless otherwise specified.



Replication cycle

  1. Influenza viruses bind to the respiratory tract epithelium.
  2. Viral hemagglutinin (H) binds sialic acid residues (neuraminic acid derivatives) on the host cell membranevirus fusion with the membrane → entry into the cell
  3. The virus replicates in the nucleus of the cell.
  4. The new virus particles travel to the cell membrane → formation of a membrane bud around the virus particles (budding)
  5. Viral neuraminidase (N) cleaves the neuraminic acidvirions exit the cell
  6. Host cell dies → cellular breakdown triggers a strong immune response

Genetic mutations

  • Antigenic shift
    • Two subtypes of viruses (e.g., human and swine influenza) infect the same cell and exchange genetic segments (reassortment) to create new subtypes (e.g., H3N1 → H2N1).
    • Occurs in particular when human pathogenic and animal pathogenic influenza viruses exchange genetic information
    • Causes pandemics (limited to a specific time period)
  • Antigenic drift

Small shifts in a panda's habitat can cause epic dread: Shifts can cause pandemics and drifts cause epidemics.


The clinical presentation of influenza infection is asymptomatic or mild in 75% of cases. Influenza presents with very characteristic features, hence the term “flu-like symptoms”.


  • RT-PCR: preferred diagnostic tool for the detection of influenza virus infection in both outpatients and hospitalized patients [10]
  • Rapid antigen test: should only be used if more sensitive diagnostics (e.g., RT-PCR) are not available
    • Used for early diagnosis
    • Detection of various influenza A/B antigens via nasal or pharyngeal swabs
    • High specificity, limited sensitivity
  • Blood tests
  • Serological testing (e.g., via complement fixation)
    • Used to diagnose an infection after it has resolved; should not be used for primary diagnosis
    • Infection is likely if serum antibody titers increase 4-fold within 1–2 weeks after acute illness

Bacterial superinfection in influenza may lead to a strong elevation in inflammation markers.

Supportive therapy

Antiviral therapy

  • Indications
    • Patients with severe disease or patients at risk of developing complications (see “Complications” below)
    • Sometimes considered if there is a high suspicion of early disease (e.g., prodromal symptoms and recent exposure)
  • Administration: should be initiated as soon as possible (within the first 48 hours)
  • Neuraminidase inhibitors: inhibit the release of viruses from the host cell.

Amantadine is no longer recommended for treating influenza! It used to be an option for treating influenza A. But during the 2008/2009 flu season, the CDC found that 100% of seasonal H3N2 and 2009 pandemic viruses were resistant to amantadine.

High-risk groups for complications

  • Elderly individuals ≥ 65 years of age
  • Children < 5 years of age, especially < 2 years of age
  • Pregnant women (and women up to two weeks after giving birth)
  • Individuals with chronic medical conditions (e.g., asthma; , heart disease, diabetes mellitus; , immunocompromise)
  • Nursing home residents
  • Native American

Most severe and dangerous complication

Other complications


We list the most important complications. The selection is not exhaustive.

  • Mortality
    • Increased in patients at high risk for influenza-related complications (see “Complications” above)
    • Average number of annual influenza-related deaths in the US: 23,000 to 48,000


Influenza vaccine


  • Hand hygiene
    • Wash hands with soap and water before and after each patient contact
    • Alternatively, use an alcohol-based hand rub
  • Avoid contact with infected individuals and stay home when sick
  • Adhere to standard precautions
    • Surface cleaning with alcohol- or aldehyde-based agents
    • Face mask
    • Gloves and gown for contact with potentially infectious material


  • Antiviral medications may be considered in patients with exposure to an infected person under certain circumstances (e.g., high risk of complications, contraindications for the vaccine, and influenza outbreaks in nursing homes).
  • For recommended agents, see “Treatment” above.


  • Definition: a type of influenza that originates from animal strains
Most common types of zoonotic influenza
Avian flu [21] Swine flu [22]
  • Direct or indirect contact with infected birds or their saliva/feces
  • Direct or indirect contact with infected pigs or their saliva/feces
Clinical features
  • See “Clinical features” above.
  • Avoid touching animals directly.
  • Always wash hands after being near animals.
  • Stay away from animals that show symptoms of a cold or flu.

Influenza during pregnancy

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