Clinical meaning
Influenza is an acute respiratory illness caused by influenza viruses, which are RNA viruses belonging to the Orthomyxoviridae family. Three types cause disease in humans: influenza A (the most clinically significant, responsible for pandemics), influenza B (causes seasonal epidemics but generally milder), and influenza C (causes mild upper respiratory illness). The influenza virus structure includes two critical surface glycoproteins: hemagglutinin (H) binds to sialic acid receptors on respiratory epithelial cells, facilitating viral entry into host cells, while neuraminidase (N) cleaves sialic acid residues to release newly formed viral particles from infected cells, enabling spread to adjacent cells. Influenza A viruses are classified by their H and N subtypes (for example, H1N1 and H3N2 are the current circulating subtypes). Two key mechanisms drive influenza evolution and epidemic behavior. Antigenic drift involves gradual accumulation of point mutations in the H and N genes during viral replication, producing minor changes in surface proteins that allow the virus to partially evade existing immunity. Antigenic drift is the reason seasonal influenza vaccines must be reformulated annually. Antigenic shift is a major, abrupt change in the H or N proteins that occurs when two different influenza A subtypes co-infect the same cell (often in an animal host such as pigs, which have receptors for both avian and human influenza) and exchange genetic segments through reassortment. Antigenic shift creates a novel virus to which the human population has little or no pre-existing immunity, potentially causing a pandemic. After inhalation of respiratory droplets or contact with contaminated surfaces followed by touching the eyes, nose, or mouth, the virus attaches to and penetrates respiratory epithelial cells of the nasopharynx and tracheobronchial tree. Viral replication destroys ciliated epithelial cells, stripping the mucociliary escalator defense mechanism and creating susceptibility to secondary bacterial infection. The host immune response includes release of pro-inflammatory cytokines (TNF-alpha, IL-6, IL-1, interferons), which are responsible for systemic symptoms (fever, myalgia, headache, fatigue) -- these systemic symptoms reflect the immune response rather than direct viral invasion of distant tissues.