How Influenza Affects the Body: From Viral Entry to Systemic Symptoms

Influenza (flu) is an acute respiratory illness caused by influenza A/B viruses. Here's the pathophysiological cascade:

1. Viral Entry & Replication

• Inhaled droplets → respiratory epithelium (nose, trachea, bronchi)
• Hemagglutinin (HA) binds sialic acid receptors → endocytosis
• Viral RNA released → nucleus → replication/transcription → assembly → budding (neuraminidase/NA releases virions)
• Peak viral load: 24–72 hours post-infection

2. Direct Viral Damage

• Ciliary dysfunction, epithelial necrosis, desquamation
• Loss of mucociliary clearance → secondary bacterial invasion
• Symptoms: rhinorrhea, sore throat, cough, hoarseness

3. Immune Response & Cytokine Storm

• Innate immunity: TLRs, RIG-I detect viral RNA → IFN-α/β, TNF-α, IL-1, IL-6
• Adaptive: CD8+ T cells clear infected cells; antibodies (anti-HA/NA) neutralize virus
• Systemic symptoms from cytokines: Fever, myalgia, fatigue, anorexia, headache (not direct viral effect)

4. Complications

• Primary viral pneumonia: Diffuse alveolar damage (high mortality)
• Secondary bacterial pneumonia: S. pneumoniae, S. aureus, H. influenzae (common cause of death)
• Exacerbations: Asthma, COPD, heart failure
• Extra-respiratory: Myositis, myocarditis, encephalitis, Reye's syndrome (aspirin in kids), GBS

5. Transmission & Duration

• Infectious: 1 day before → 5–7 days after symptoms (longer in kids/immunocompromised)
• Typical illness: 3–7 days; cough/fatigue may persist 2+ weeks

High-Risk Groups

≥65 yrs, <5 yrs, pregnant, chronic diseases (lung, heart, metabolic, immune), nursing home residents.

Prevention & Treatment

Annual vaccination (best protection). Antivirals (oseltamivir, baloxavir) if started ≤48h reduce severity/duration. Supportive care: fluids, rest, antipyretics.