influenzavirus

Influenzavirus is a genus of enveloped, segmented, negative-sense RNA viruses in the family Orthomyxoviridae. The genus comprises four species: Influenzavirus A, B, C, and D. Influenza A and B are the primary cause of seasonal influenza in humans; influenza C generally causes milder respiratory illness; influenza D has been identified in cattle and other animals and is rarely found in humans.

Virion and genome characteristics vary by species. Influenza A and B viruses display two major surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), which define numerous subtypes (for example, H1N1, H3N2). Influenza C and D use a single surface protein, hemagglutinin-esterase-fusion (HEF). The genome is segmented and consists of 7 (C and D) or 8 (A and B) negative-sense RNA segments. The virus replicates in the host cell nucleus and employs cap-snatching to initiate transcription.

Transmission and disease ecology: Influenza viruses spread mainly via respiratory droplets and close contact. In humans, infections range from mild upper-respiratory illness to severe disease and pneumonia, with higher risk for young children, older adults, and those with certain health conditions. Seasonal epidemics arise annually, while rare reassortment events can produce pandemic strains.

Antigenic variation and evolution: Antigenic drift refers to gradual mutations in HA and NA that allow escape from host immunity, driving yearly vaccine updates. Antigenic shift involves reassortment of genome segments, potentially creating novel viruses with pandemic potential, particularly among influenza A viruses that infect multiple species.

Prevention and treatment: Annual vaccines target circulating A and B strains and are available as inactivated, live attenuated, or recombinant formulations. Antiviral agents include neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) and the cap-dependent endonuclease inhibitor baloxavir. Surveillance informs vaccine strain composition and public health responses.