Influenza

Definition

Influenza is an acute respiratory infection caused by influenza viruses A, B, or C, characterized by fever, cough, myalgia, and fatigue. It can lead to complications such as pneumonia, exacerbation of chronic diseases, and hospitalization in high-risk groups.

Epidemiology

• Seasonal outbreaks occur annually, typically in winter in temperate climates and year-round in tropical regions
• Influenza A causes most epidemics and pandemics; Influenza B usually causes milder seasonal outbreaks
• All age groups are susceptible, with higher morbidity and mortality in the elderly, young children, pregnant women, and immunocompromised individuals
• Vaccination coverage influences population-level incidence and severity
• Global burden: hundreds of thousands of deaths annually, primarily from complications

Etiology

• Influenza viruses: Orthomyxoviridae family; types A, B, C (A and B clinically significant)
• Influenza A subtypes determined by hemagglutinin (H) and neuraminidase (N) surface proteins
• Transmission via respiratory droplets, aerosols, and contaminated surfaces
• Risk factors: crowded living conditions, poor hygiene, chronic medical conditions, pregnancy, immunosuppression
• Antigenic drift (minor mutations) and antigenic shift (major changes) contribute to seasonal epidemics and pandemics

Pathophysiology

• Virus infects respiratory epithelial cells, causing cell death and desquamation
• Local inflammation leads to nasal congestion, sore throat, and cough
• Systemic cytokine response causes fever, myalgia, malaise, and fatigue
• Secondary bacterial infection may occur due to disrupted mucosal barriers
• Severe infection can cause viral pneumonia, acute respiratory distress syndrome (ARDS), and multi-organ involvement