Any member of the Orthomyxoviridae family with a segmented negative‑sense RNA genome that causes influenza, a contagious respiratory disease in humans and other animals.
Explanation
Influenza viruses are enveloped viruses with a helical nucleocapsid containing eight (types A and B) or seven (type C) genome segments. They are classified into types A, B, C and D based on internal proteins. Type A viruses infect humans, birds and other mammals and are further subtyped by the antigenic properties of their surface glycoproteins haemagglutinin (H) and neuraminidase (N); examples include H1N1 and H3N2. The segmented genome allows reassortment, enabling antigenic shift when two different viruses infect the same cell and exchange gene segments. Along with continual antigenic drift from point mutations, this genetic variability drives seasonal epidemics and occasional pandemics. Type B viruses primarily infect humans and seals and cause seasonal outbreaks but lack the same pandemic potential. Type C viruses cause mild respiratory illness, and type D mainly affects cattle.
Influenza viruses attach to sialic acid receptors on respiratory epithelial cells via haemagglutinin, are endocytosed and undergo uncoating; replication and transcription occur in the host cell nucleus, and progeny virions are released with the help of neuraminidase. Transmission occurs through respiratory droplets, aerosols and contaminated surfaces. Symptoms include fever, cough, sore throat, myalgia and malaise; complications can include viral pneumonia, secondary bacterial infections and worsening of chronic conditions. Vaccination with inactivated or live attenuated influenza vaccines is updated annually to match circulating strains. Antiviral drugs such as oseltamivir inhibit neuraminidase to reduce disease severity when administered early.
Notable subtypes and events
The 1918 H1N1 influenza A pandemic caused severe mortality worldwide, illustrating the impact of antigenic shift. The 2009 H1N1 pandemic arose from reassortment of swine, avian and human influenza viruses and led to widespread illness but lower mortality than expected. Seasonal influenza A(H3N2) strains have predominated in many recent seasons, particularly affecting older adults. Highly pathogenic avian influenza A(H5N1) and A(H7N9) viruses cause severe disease in poultry and sporadic human infections with high case fatality rates. Influenza B viruses from the Victoria and Yamagata lineages circulate in humans and are included in quadrivalent vaccines.
Awareness of influenza virus diversity and evolution guides surveillance, vaccine composition and preparedness for future outbreaks.
Related Terms: Influenza, Orthomyxovirus, Antigenic drift, Hemagglutinin, Vaccination