Influenza virus- Structure, Types, Life Cycle

Influenza commonly called a flue is a highly contagious viral infection of the respiratory tract mainly infects humans and animals.

  • Influenza virus belongs to the family: Orthomyxoviridae
  • The annual attack rate of influenza virus is 5-10% in adults and 20-30% in children.

Virus Structure

  • Enveloped virus
  • Has helical nucleocapsid
  •  sense segmented single-stranded RNA
  • Viruses are spherical in shape
  • Their size varies from 80-180 nm

Influenza virus- Structure, Types, Life Cycle

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Types of influenza virus

There are three main types of influenza virus :

1. Influenza A

  • Most virulent human pathogen.
  • Capable of infecting humans, chickens, and pigs.
  • Type A viruses are divided on the basis of the difference in a viral surface protein called Haemagglutinin and Neuraminidase.
  • Influenza virus undergoes two kinds of changes :
    • antigenic drift
    • antigenic shift
  • There are two subtypes of influenza A virus :
    • H5N1, H1N1.

2. Influenza B

  • Influenza B viruses change only by the more gradual process of antigenic drift.
  • Mainly infects humans.
  • Causes epidemic disease similar to type A.

3. Influenza C

  • Influenza C virus usually causes mild disease in children.
  • This type of virus lack NA and contain esterase.
  • Type C viruses are less common than other types.

Life Cycle Of Influenza Virus


  • The entry of vRNPs into the nucleus
  • Transcription and replication of the viral genome
  • Export of vRNP from the nucleus
  • Assembly and budding at the host cell plasma membrane

Signs And Symptoms

  • Fever
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Headache
  • Muscle aches
  • Extreme fatigue

Mode of Transmission

  • Spread from person to person by droplet infection created by sneezing, coughing and talking.
  • The portal of entry is the respiratory tract.


  • The virus infects both upper and lower respiratory tracts.
  • Sialic acid on epithelial cells are their receptors.
  • The typical incubation period is 24 hrs to 4 days.


  • Increase liquid intake, warm shower.
  • Warm compresses, especially in the nasal area can reduce nasal congestion.



  • 1: flu shot – an inactivated vaccine that is given with needle in the arm
  • 2: nasal spray – a vaccine made with live weakened flu viruses that are given as nasal spray.



Influenza virus- Structure, Types, Life Cycle

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