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Influenza

Clinical Subject Page

Influenza

Influenza (flu) is an acute contagious viral infection of the respiratory tract caused by influenza viruses, typically producing sudden fever, cough, headache, and prominent systemic symptoms.

Also called

Flu

ICD-10

J11.1

Specialty

Pulmonology

Onset

Acute

Reviewed

July 2026

On This Page

Overview

It mainly affects the:

  • Nose
  • Throat
  • Airways
  • Lungs

Unlike the common cold, it usually causes a more sudden and severe systemic illness, with prominent:

  • Fever
  • Myalgia
  • Headache
  • Fatigue

Most patients recover completely, but severe disease may cause pneumonia, respiratory failure, or death, particularly in high-risk patients.

Etiology & Risk Factors

Influenza A

  • Causes seasonal epidemics
  • Can cause pandemics
  • Infects humans and some animals

Influenza B

  • Causes seasonal epidemics
  • Mainly affects humans

Transmission

spreads through:

  • Respiratory droplets and particles
  • Close contact with infected people
  • Contact with contaminated respiratory secretions

Risk Factors for Severe Disease

  • Age ≥65 years
  • Young children
  • Pregnancy
  • Chronic lung disease
  • Cardiovascular disease
  • Diabetes mellitus
  • Chronic kidney disease
  • Immunosuppression
  • Severe obesity

Important Note

 A + B are the main causes of clinically significant seasonal disease.

Pathophysiology

Exposure to the virus
⬇
Virus enters the respiratory tract
⬇
Attachment to respiratory epithelial cells
⬇
Viral replication
⬇
Epithelial cell injury
⬇
Strong inflammatory and immune response
⬇
Airway inflammation + Systemic cytokine release
⬇
Fever + Myalgia + Headache + Fatigue + Respiratory symptoms

Severe Disease

Extensive lung involvement
→ Viral pneumonia
→ Impaired gas exchange
→ Respiratory failure

Key Concept

The strong systemic inflammatory response explains the prominent fever, myalgia, headache, and fatigue.

Clinical Presentation

  • Typical Symptoms

    • Sudden fever
    • Chills
    • Dry cough
    • Sore throat
    • Headache
    • Myalgia
    • Marked fatigue
    • Malaise
    • Nasal symptoms

    Typical Pattern

    Symptoms usually:

    • Begin suddenly
    • Are more severe than the common cold
    • Include prominent systemic symptoms

    Signs

    • Fever
    • Tachycardia
    • Throat redness
    • Nasal discharge

    Severe Disease

    May present with:

    • Dyspnea
    • Hypoxemia
    • Chest pain
    • Confusion
    • Respiratory distress

History Taking

  • Ask about:

    • Sudden onset of symptoms?
    • Fever or chills?
    • Dry cough?
    • Sore throat?
    • Headache?
    • Muscle aches?
    • Severe fatigue?
    • Shortness of breath?
    • Chest pain?
    • Sick contacts?
    • Recent outbreak?
    • vaccination history?

Physical Examination

Look for:

  • Fever
  • Tachycardia
  • General illness
  • Dehydration
  • Reduced oxygen saturation
  • Respiratory distress

Nose & Throat Examination

May show:

  • Nasal discharge
  • Throat redness

Chest Examination

Usually normal in uncomplicated cases.

May show:

  • Crackles
  • Reduced air entry
  • Signs of pneumonia

Investigations

  • Routine Testing

    Testing is not always required in mild, uncomplicated disease.

    Molecular Testing — Preferred Test

    • Rapid molecular assays
    • RT-PCR

    These tests detect influenza viral RNA.

    When Testing Is Most Useful

    Consider testing when:

    • The patient is hospitalized
    • Severe disease is present
    • The patient is at high risk of complications
    • The result will change treatment or infection-control decisions

    Chest X-ray

    Not routinely required.

    Consider when:

    • Pneumonia is suspected
    • Hypoxemia is present
    • Symptoms are severe or worsening

    Additional Tests

    In severe disease:

    • CBC
    • Renal function
    • Electrolytes
    • Liver function
    • Arterial blood gas when indicated

Diagnosis

  • Diagnosis is based on:

    1. Compatible Clinical Features

    • Sudden fever
    • Cough
    • Myalgia
    • Headache
    • Fatigue

    2. Epidemiological Context

    • the disease circulating in the community
    • Contact with infected individuals

    3. Viral Testing When Indicated

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Related Topics

Management of Influenza

  • Main Goals

    • Relieve symptoms
    • Reduce complications
    • Treat high-risk patients early
    • Prevent transmission

    Supportive Management

    • Rest
    • Adequate fluids
    • Analgesics
    • Antipyretics
    • Oxygen when hypoxemic

    Antiviral Treatment

    Oseltamivir

    A commonly used antiviral treatment.

    Other antivirals may be used depending on:

    • Patient factors
    • Disease severity
    • Local recommendations

    Antiviral treatment is particularly important for:

    • Hospitalized patients
    • Severe or progressive disease
    • High-risk patients

    Timing

    Treatment is most effective when started early, ideally within 48 hours of symptom onset, but may still benefit hospitalized or severely ill patients when started later.

    Antibiotics

    • Not routinely indicated
    • because it is a viral infection
    • Use only when bacterial coinfection is suspected or confirmed

Complications of Influenza

  • Primary viral pneumonia
  • Secondary bacterial pneumonia
  • Respiratory failure
  • Acute respiratory distress syndrome (ARDS)
  • Asthma exacerbation
  • COPD exacerbation
  • Myocarditis
  • Pericarditis
  • Encephalitis

Prognosis of Influenza

  • Most healthy patients recover completely.
  • Symptoms usually improve over several days.
  • Fatigue and cough may persist longer.
  • Prognosis is worse in:
    • Older adults
    • Young children
    • Pregnant patients
    • Immunocompromised patients
    • Patients with chronic diseases
  • Severe disease may cause pneumonia, respiratory failure, and death.

Key Points / Clinical Pearls of Influenza

  • Influenza is an acute respiratory infection caused mainly by influenza A and B viruses.
  • Symptoms usually begin suddenly.
  • Fever, dry cough, myalgia, headache, and marked fatigue are classic features.
  • Systemic symptoms are usually more prominent than in the common cold.
  • Molecular testing is preferred when confirmation is needed.
  • Do not delay antivirals in severely ill or high-risk patients while awaiting results.
  • Antivirals work best when started early.
  • Antibiotics do not treat uncomplicated cases.