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 and mouth when sneezing and regularly washing your hands can help prevent the flu from spreading.2  

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? 
Use our symptoms checker

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
  • Pneumonia
  • Inflammation of the heart
  • Multiple organ failure
  • Inflammation of the muscle
Sick woman

In Singapore, seasonal influenza infects about 20% of the population, resulting in up to 600 deaths every year.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: February 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: February 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: February 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. Ministry of Home Affairs (MHA). Preparing for a human influenza pandemic in Singapore. Available from: www.mha.gov.sg/docs/default-source/others/nsfpfinalversion.pdf. Last accessed: February 2020.