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