Influenza (Flu - Influenza Viruses)

Overview

Plain-Language Overview

Influenza (Flu) is a contagious respiratory illness caused by influenza viruses that primarily affect the nose, throat, and lungs. It spreads easily from person to person through droplets when an infected person coughs or sneezes. The main symptoms include fever, cough, sore throat, body aches, and fatigue, which can range from mild to severe. The flu can lead to complications such as pneumonia or worsen chronic health problems, especially in young children, older adults, and people with weakened immune systems. Each year, seasonal flu outbreaks cause widespread illness and can significantly impact daily activities and healthcare systems.

Clinical Definition

Influenza (Flu) is an acute viral infection of the respiratory tract caused by influenza A, B, or C viruses, with types A and B being the most clinically significant in humans. The virus infects the epithelial cells of the upper and lower respiratory tract, leading to inflammation and cell death. Transmission occurs primarily via respiratory droplets and contact with contaminated surfaces. The infection triggers a systemic immune response characterized by fever, myalgia, and malaise, alongside respiratory symptoms such as cough and sore throat. The disease is significant due to its potential for seasonal epidemics and pandemics, causing substantial morbidity and mortality worldwide. Complications include secondary bacterial pneumonia, exacerbation of chronic diseases, and acute respiratory distress syndrome (ARDS) in severe cases.

Inciting Event

Inhalation of aerosolized respiratory droplets containing influenza virus from infected persons.
Contact with contaminated surfaces followed by self-inoculation of nasal or conjunctival mucosa.
Seasonal outbreaks typically begin in fall or winter months in temperate climates.

Latency Period

The incubation period is typically 1 to 4 days after exposure to influenza virus.
Symptoms usually develop rapidly within 24 to 48 hours of viral entry.

Diagnostic Delay

Early symptoms mimic common viral illnesses, leading to misattribution as common cold.
Lack of rapid testing availability or use delays confirmation of influenza infection.
Mild or atypical presentations in elderly or immunocompromised patients cause underrecognition.

Clinical Presentation

Signs & Symptoms

Sudden onset of high fever with chills is a hallmark symptom.
Myalgia and arthralgia cause significant body aches.
Nonproductive cough and sore throat are common respiratory symptoms.
Headache and fatigue often accompany systemic illness.
Nasal congestion and rhinorrhea may be present but are less prominent than in common cold.

History of Present Illness

Abrupt onset of high fever, chills, and malaise marks the initial presentation.
Prominent myalgias, headache, and nonproductive cough develop within hours to days.
Symptoms peak within 2 to 3 days and typically resolve over 5 to 7 days in uncomplicated cases.
Some patients experience sore throat, nasal congestion, and fatigue during illness.
Severe cases may progress to dyspnea and hypoxia indicating lower respiratory tract involvement.

Past Medical History

History of chronic pulmonary or cardiovascular disease increases risk of complications.
Previous influenza vaccination status affects susceptibility and disease severity.
Immunocompromising conditions such as HIV infection or chemotherapy worsen outcomes.
Prior episodes of influenza or other respiratory infections may influence immune response.

Family History

No specific heritable syndromes are associated with influenza susceptibility.
Family members often share exposure risk due to close contact and household transmission.
Genetic polymorphisms in immune response genes may modulate severity but are not routinely tested.

Physical Exam Findings

Fever and tachycardia are common during acute influenza infection.
Pharyngeal erythema and nasal congestion are frequent upper respiratory findings.
Diffuse crackles or rhonchi may be heard in cases with lower respiratory involvement.
Conjunctival injection can be present in some patients.
Lymphadenopathy is usually mild or absent.

Diagnostic Workup

Diagnostic Criteria

Diagnosis is primarily clinical, based on the presence of acute onset fever, cough, and respiratory symptoms during influenza season. Confirmation is achieved by detecting viral RNA or antigens using reverse transcription polymerase chain reaction (RT-PCR) or rapid antigen tests from nasopharyngeal swabs. Viral culture is less commonly used due to longer turnaround time. Chest imaging may be performed to evaluate for complications but is not diagnostic of influenza itself. Laboratory findings are nonspecific but may show leukopenia or mild lymphopenia.

Pathophysiology

Key Mechanisms

Infection of respiratory epithelial cells by influenza viruses leads to cell death and impaired mucociliary clearance.
Viral hemagglutinin (HA) mediates attachment and entry into host cells, initiating infection.
Immune response activation causes systemic symptoms and local inflammation in the respiratory tract.
Cytokine release syndrome contributes to fever, malaise, and myalgias during acute illness.
Antigenic drift and shift enable viral evasion of host immunity and seasonal epidemics or pandemics.

Involvement	Details
Organs	Lungs are the primary organs affected, with viral pneumonia as a major complication.
	Nasal passages serve as the initial site of viral entry and replication.
	Spleen participates in systemic immune response and clearance of infected cells.
Tissues	Respiratory mucosa is damaged by viral replication leading to inflammation and increased susceptibility to secondary bacterial infections.
Cells	Respiratory epithelial cells are the primary site of influenza virus infection and replication.
	Cytotoxic CD8+ T cells mediate clearance of infected cells during the immune response.
	Macrophages contribute to viral clearance and release proinflammatory cytokines causing symptoms.
Chemical Mediators	Interferon-alpha is produced by infected cells and initiates antiviral defenses.
	Tumor necrosis factor-alpha (TNF-α) contributes to systemic symptoms like fever and malaise.
	Interleukin-6 (IL-6) promotes acute phase response and fever during infection.

Treatments

Pharmacological Treatments

Oseltamivir
- Mechanism:
  - Inhibits the neuraminidase enzyme, preventing viral release from infected cells.
- Side effects:
  - Nausea
  - Vomiting
  - Headache
- Clinical role:
  - First-line
Zanamivir
- Mechanism:
  - Blocks neuraminidase, reducing viral spread in the respiratory tract.
- Side effects:
  - Bronchospasm
  - Cough
  - Nasal irritation
- Clinical role:
  - First-line
Baloxavir marboxil
- Mechanism:
  - Inhibits the cap-dependent endonuclease activity of the viral polymerase, blocking viral mRNA synthesis.
- Side effects:
  - Diarrhea
  - Bronchitis
  - Headache
- Clinical role:
  - Second-line

Non-pharmacological Treatments

Supportive care with hydration and rest to aid recovery.
Use of antipyretics such as acetaminophen to reduce fever and myalgia.
Isolation and respiratory hygiene to prevent transmission.

Prevention

Pharmacological Prevention

Annual influenza vaccination with inactivated or live-attenuated vaccines is the primary prevention.
Neuraminidase inhibitors such as oseltamivir can be used for post-exposure prophylaxis in high-risk individuals.
Chemoprophylaxis is reserved for unvaccinated or severely immunocompromised patients during outbreaks.

Non-pharmacological Prevention

Hand hygiene with soap and water or alcohol-based sanitizers reduces transmission.
Respiratory etiquette including covering coughs and sneezes limits spread.
Avoiding close contact with infected individuals during peak influenza season is recommended.
Use of masks in crowded or healthcare settings can reduce viral spread.
Environmental cleaning of frequently touched surfaces decreases fomite transmission.

Outcome & Complications

Complications

Primary viral pneumonia causing respiratory failure is a severe complication.
Secondary bacterial pneumonia often due to Staphylococcus aureus or Streptococcus pneumoniae.
Exacerbation of chronic lung diseases such as asthma or COPD.
Myocarditis and pericarditis can occur rarely.
Reye syndrome is a rare complication in children given aspirin during influenza.

Short-term Sequelae	Long-term Sequelae
Post-viral cough lasting weeks after acute illness. Fatigue and malaise persisting beyond fever resolution. Sinusitis or otitis media as secondary infections. Transient worsening of chronic diseases such as heart failure or asthma.	Chronic respiratory impairment may develop after severe viral pneumonia. Rarely, neurological complications such as Guillain-Barré syndrome may occur. No direct long-term sequelae in most healthy individuals.

Differential Diagnoses

Influenza (Flu - Influenza Viruses) versus Common Cold (Rhinovirus Infection)

Influenza (Flu - Influenza Viruses)	Common Cold (Rhinovirus Infection)
Abrupt onset with high fever and systemic symptoms lasting about 3-7 days	Gradual onset with mild symptoms lasting 7-10 days
Leukopenia with relative lymphocytosis common	Normal or mildly elevated white blood cell count without lymphopenia
Exposure during influenza season with outbreaks in winter	Exposure to individuals with mild upper respiratory symptoms, often in fall and spring

Influenza (Flu - Influenza Viruses) versus Respiratory Syncytial Virus (RSV) Infection

Influenza (Flu - Influenza Viruses)	Respiratory Syncytial Virus (RSV) Infection
Affects all ages but more severe in elderly and immunocompromised	Primarily affects infants and young children under 2 years
Predominantly upper respiratory symptoms with systemic fever and myalgias	Prominent lower respiratory tract symptoms with wheezing and bronchiolitis
Positive rapid antigen or PCR test for influenza virus	Positive rapid antigen or PCR test for RSV

Influenza (Flu - Influenza Viruses) versus Bacterial Pneumonia

Influenza (Flu - Influenza Viruses)	Bacterial Pneumonia
Acute onset with dry cough, systemic symptoms, and diffuse respiratory signs	Progressive symptoms with productive cough, pleuritic chest pain, and localized findings
Leukopenia or normal WBC count, procalcitonin usually normal	Leukocytosis with neutrophilia and elevated procalcitonin
Diffuse or patchy interstitial infiltrates or normal chest X-ray early in illness	Lobar consolidation on chest X-ray

Influenza (Flu - Influenza Viruses) versus COVID-19 (SARS-CoV-2 Infection)

Influenza (Flu - Influenza Viruses)	COVID-19 (SARS-CoV-2 Infection)
Exposure during influenza season without known COVID-19 contact	Known contact with confirmed COVID-19 case or high community prevalence
Short incubation with abrupt onset of fever, chills, and myalgias	Variable incubation period with symptoms ranging from mild to severe respiratory distress
Positive influenza virus PCR or rapid antigen test	Positive SARS-CoV-2 PCR or antigen test

Influenza (Flu - Influenza Viruses) versus Acute Bronchitis

Influenza (Flu - Influenza Viruses)	Acute Bronchitis
Abrupt onset of high fever and systemic symptoms with dry cough	Cough lasting more than 5 days, often with sputum production, without high fever
Leukopenia and elevated inflammatory markers common	Normal or mildly elevated inflammatory markers, no leukopenia
Antiviral treatment effective if started early	Symptomatic treatment; antibiotics rarely indicated