Last edited 05/2022 and last reviewed 05/2022

Influenza or “flu” is an upper respiratory tract infection caused by a virus belonging to the Orthomyxoviridae family (1).

There are three serotypes – A, B and C – out of which A and B are responsible for epidemics of respiratory illness:

  • type A
    • causes outbreaks in most years and is the usual cause of epidemics (2)
    • is further divided into subtypes according to the two proteins on the external coat , the hemagglutinin (H1–H18) and the neuraminidase proteins (N1–N11) (3)
  • type B
    • causes less severe disease and milder outbreaks (2)
    • but in children the severity may be similar to that seen in influenza A infection (2)
    • generally responsible for regional outbreaks (4)
  • type C
    • causes a milder infection that does not cause epidemics

Seasonal influenza consists at present of variable mixes of type A and B influenza viruses (3).

Epidemics of influenza occur mainly in the winter months. Profound changes in the viral antigens result in larger outbreaks called pandemics.

Influenza is usually a self-limiting disease but significant mortality is observed in those with chronic chest or cardiovascular disease.


  1. World Health Organization (WHO). WHO Global Influenza Surveillance Network. Manual for the laboratory diagnosis and virological surveillance of influenza  
  2. Immunisation Against Infectious Disease - "The Green Book". Chapter 19. Influenza (October 2020)  
  3. European Centre for Disease Prevention and Control (ECDC). Factsheet for professionals on seasonal influenza  
  4. National Travel Health Network and Centre (NaTHNaC) 2021. Influenza (flu) factsheet.