Influenza Infection

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...