Overview of the influenza virus

Transmission electron micrograph of influenza H1N1 virionsInfluenza viruses are responsible for the highly contagious flu disease that can range in severity from mild to serious illness. This virus is important globally. For centuries influenza infections have caused both epidemics (the rapid spread of infectious disease to a large number of people) and pandemics (the worldwide spread of a new disease). In this article we will discuss the classification, structure and versatility of the influenza virus.

Classification and structure of the influenza virus

3D representation of influenza structure

The influenza virus belongs to the Orthomyxoviridae family, which is subdived into six genera. Of these genera influenza A, influenza B and influenza C are the most relevant to humans and animals. Influenza A and C infect multiple species, while influenza B almost exclusively infects humans [1]. Influenza viruses are enveloped RNA viruses approximately 100 nm in diameter. The viral nucleic acid is contained in a protein shell, which in turn is surrounded by a lipid bilayer that holds viral proteins. Two surface glycoproteins characterise influenza’s surface: heamagglutinin and neuraminidase. Influenza A is divided into subtypes on the basis of serological and genetic differences in these two surface glycoproteins. There are 15 subtypes of hemagglutinin (H1 to H15) and 9 subtypes of neuraminidase (N1 to N9).
So Influenza A H1N1, the cause of the 2009 swine flu pandemic, is characterised by the hemagglutinin subtype H1 and the neuraminidase subtype N1. In addition to that 2009 outbreak, since 1900 there have been three other recorded influenza pandemics: the H1N1 Spanish Flu in 1918, the H2N2 Asian Flu (1957) and the Hong Kong pandemic (1968).

What makes Influenza A so versatile and epidemiologically successful?

The influenza A virus can undergo genetic changes, which make a person susceptible to the virus even if they were previously infected by ‘old’ virus strains. These continuous genetic modifications can be caused by (a) accumulation of point mutations in heamagglutinin and neuraminidase, (b) recombination between different viral strains (this is not common) and (c) genetic reassortment (this only occurs for influenza A). The greater the change in the heamagglutinin and neuraminidase surface proteins, the less likely it is that your immune system will recognise the virus.

Another factor contributing to the versatility of the virus is its ability to be transmitted from animals to humans. Such diseases are known as zoonoses. Migratory aquatic birds around the world are a major influenza A virus reservoir. Occasionally the virus can jump host species barriers and infect other animals such as marine mammals, pigs and domestic poultry. This phenomenon is crucial, because it brings the virus closer to humans. Humans are more frequently exposed to pigs and poultry than we are to wild birds. Zoonotic events are generally restricted. However, in some cases influenza viruses have acquired the ability to be transmitted from animals to humans, and then between humans. Pandemics can occur as a consequence of such events.

How is the influenza virus transmitted?

Salivary droplets from sneeze

The main Influenza virus transmission routes are by contact and through the air. Infection can occur due to direct or indirect contact with contaminated environments or by droplets generated by an infected person. Large droplets released into the air can turn into small particle droplet nuclei [2]. These smaller particles can travel greater distances and persist longer in the air. Virus persistence in the air is dependent on the prevailing conditions, which in an indoor environment are mainly the level of aeration, temperature and humidity.
The influenza virus was demonstrated to be infective for up to a few hours when it is deposited on non-porous surfaces in indoor environments [3, 4].  But more importantly public building air filters, aeroplanes, health centre and day care facilities have been found to be positive for airborne Influenza A virus [5, 6]. This shows that virus contamination is present in the air and in the environment in general.


  1. The evolution of human influenza viruses. Philosophical Transactions of the Royal Society B: Biological Sciences. 2001, 356 (1416): 1861–70.
  2. Aerosol transmission is an important mode of influenza A virus spread. Nature communications. 2013 (4) 1935. doi:10.1038/ncomms2922.
  3. Inactivation of Influenza virus on copper versus stainless steel surfaces. Applied and Environmental Microbiology. 2007, 73:2748-50.
  4. Detection of viruses on used ventilation filters from two large public buildings. American Journal of Infection Control. 2011, 39(7):30-38.
  5. Survival of Influenza A H1N1 on materials found in households: implications for infection control. PLoS ONE. 2011. 6(11):e27932.
  6. Concentrations and size distributions of airborne influenza A viruses measured indoors at a health centre, a day-care centre and on aeroplanes.  Journal of the Royal Society Interface. doi:10.1098/rsif.2010.0686

Photo Credits

  • Transmission electron micrograph of influenza H1N1 virions - Content provider: CDC/ Cynthia Goldsmith - CDC Public Health Image Library ID# 11745
  • 3D representation of influenza structure - Content provider: CDC/ Douglas Jordan. Illustrator: Dan Higgins - CDC Public Health Image Library ID# 11824
  • Salivary droplets from sneeze - Content provider: CDC/Brian Judd. Photo Credit: James Gathany - CDC Public Health Image Library ID# 11161

This article was written by Mariarita Arenella PhD. Mariarita is a virologist at airmid healthgroup.