Influenza (flu) Virus: Introduction, Classification, Structure

By Acharya Tankeshwar •  Updated: 06/06/21 •  4 min read

Influenza commonly called “the flu”, is a contagious respiratory illness, a very important global public health problem. It causes seasonal flu epidemics every year and is an important disease responsible for school and job absenteeism, hospitalization, and deaths. Flu pandemics have killed millions of people worldwide and another flu pandemic is looming in our future.

Influenza (flu) is caused by influenza viruses, a member of the orthomyxovirus family. Influenza viruses infect the respiratory tract (i.e., nose, throat, lungs) and can cause mild to severe life-threatening illnesses. There are three types of influenza (flu) viruses: A, B, and C. Antigenic differences exhibited by two of the internal structure proteins (NP and M) are used to divide influenza viruses into types A, B, and C.

Influenza virus A and B cause seasonal epidemics whereas influenza type C causes mild respiratory illness. Currently circulating influenza B viruses are from one of two lineages: B/Yamagata and B/Victoria. There are no subtypes of influenza B virus.

Influenza A virus has been further divided into subtypes and strains. The mutability and high frequency of genetic reassortment in this virus result in antigenic changes in viral surface glycoprotein which have made influenza type A antigenically highly variable and responsible for most cases of epidemic influenza.

Subtypes of Influenza A viruses

Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 18 different hemagglutinin subtypes (H1 through H18) and 11 different neuraminidase (N1 through N11) subtypes.

Strains of Influenza A viruses

Influenza A viruses can be further broken down into different strains. Some of the  most successful and virulent strains of influenza are

  1. H5N1 avian (bird) flu: caused outbreaks in domestic poultry in parts of Asia and the Middle East. Human infection with H5N1 is rare, nearly 650 cases of human cases of H5N1 have been reported from 15 countries since 2003.
  2. H1N1; originally referred to as swine flu: caused a worldwide pandemic in 2009.
  3. Influenza A H3N2 variant viruses

Nomenclature (naming) of Influenza Virus 

Internationally accepted naming convention is followed for the naming of influenza viruses. The approach uses the following components:

Naming of influenza virus
Fig: Naming of influenza virus

Strain number (e.g., 15, 7, etc.) Year of isolation (e.g., 57, 2009, etc.) For influenza A viruses, the hemagglutinin and neuraminidase antigen description in parentheses (e.g., (H1N1), (H5N1)

For example:

Structure of Influenza virus 

Influenza (flu) virus structure Image source: CDC
Influenza (flu) virus structure
Image source: CDC

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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