What is the flu?

The flu is an illness, caused by the influenza virus, that spreads when infected people cough or sneeze and release infectious virus particles into the air.1 Covering your nose when sneezing and regularly washing your hands can help prevent the flu from spreading.2 However, these measures are rarely enough.

A picture of creased tissue

The flu causes a range of different symptoms that leave people feeling very unwell including fatigue, aches and fever.3,4 

Identification of the flu can be difficult, as symptoms can also be caused by other illnesses, such as the common cold.

Suspect the flu? 
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Green tissue box

How many people does the flu affect?

Globally, up to 1 billion cases of the flu are estimated to occur each year.5 Of these, up to 5 million cases are deemed severe, resulting in up to 650,000 deaths.6

How serious
is the flu?

Many people see the flu as a minor illness with little threat of serious consequences.7–9  ­In reality, the flu can cause a wide range of severe complications – which, in some cases, can be life threatening. These include:4

  • Inflammation of the brain
  • Blood poisoning (sepsis)
  • Worsening of existing health
    problems, such as asthma
  • Inflammation
    of the muscle
  • Inflammation of the heart
  • Pneumonia
  • Multiple organ failure
Sick woman

This burden of flu is thought to be similar across tropical and subtropical countries within the Asia-Pacific region and temperate regions, including Europe.10


  1. Cowling B et al. Nat Commun 2013; 4: 1935.
  2. Centers for Disease Control and Prevention (CDC). Healthy habits to help prevent flu. Available from: www.cdc.gov/flu/prevent/actions-prevent-flu.htm. Last accessed: August 2020.
  3. Banning M. Br J Nurs 2005; 14(22): 1192–1197.
  4. Centers for Disease Control and Prevention (CDC). Flu symptoms and complications. Available from: www.cdc.gov/flu/symptoms/symptoms.htm. Last accessed: August 2020.
  5. Krammer F et al. Nat Rev Dis Primers 2018; 4(1): 3. doi: 10.1038/s41572-018-0002-y.
  6. World Health Organisation (WHO). Influenza (Seasonal). Available from:   www.who.int/news-room/fact-sheets/detail/influenza-(seasonal). Last accessed: August 2020.
  7. Evans MR et al. Br J Gen Pract. 2007; 57: 352–358.
  8. Cedraschi C et al. BMC Fam Pract 2013; 14: 15.
  9. Nowak G et al. Int J Environ Res Public Health 2018; 15(4): 711.
  10. Cowling B et al. Vaccine 2017; 35: 856–864.