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Influenza (flu) is an acute air-borne viral infection of the upper respiratory tract. The name originates from the Italian word meaning influence, as the disease was once believed to have been influenced by the stars.

The flu virus

With permission from ESWI Flucentre. Originally published in Rapid Reference to Influenza, Second Edition, Elsevier, 2006

Influenza viruses are classified into the A, B and C groups. Influenza A and B are structurally similar surrounded with glycoprotein spikes of Hemagglutinin (HA) and Neuraminidase (NA), whereas the influenza C virus has a single surface glycoprotein known as Hemagglutinin-esterase-fusion. Influenza A viruses are classified further based on their surface glycoproteins HA and NA, making up several genetically distinct subtypes. Over the last 30 years human flu epidemics have been caused by influenza A H1N1 and H3N2, and influenza B.

Group A and B viruses cause regular epidemics of flu (seasonal flu) in humans and Group A viruses occasionally cause pandemics. Influenza viruses continuously evolve through: 

 -  antigenic drift (point mutations in genes that code the surface proteins hemagglutinin and neuraminidase). This process results in new strains of flu that emerge from year to year

  -  antigenic shift (genetic reassortment, where the influenza strain acquires the hemagglutinin and neuraminindase segment from an influenza virus of a different subtype). This occurs very occasionally and can result in a pandemic if the new virus is transmissible in humans. This occurred 3 times in the 20th century. On the 11th of June 2009 the WHO declared a new global pandemic of a new variant of H1N1 (swine flu), which was first isolated in humans in Mexico in the spring of 2009.


Direct human-to-human transmission occurs by ingestion/inhalation of contaminated respiratory secretions. When an infected individual coughs/sneezes the virus escapes via aerosols thereby enabling a healthy individual to become infected when he comes into contact with the contaminated air or surface. Using a handkerchief can transfer virus to the hands, and so it can be passed on by touch.


Most people recover from influenza within 1-2 weeks. Infection is often characterised by

  • Chills  
  • Sudden onset of high fever (38 - 39 ºC)  
  • Headache  
  • Cough  
  • Sore throat  
  • Runny nose
  • Muscle pain

Death from influenza can occur, usually in the elderly and frail. It is often associated with secondary bacterial infections caused by Staphylococcus aureus, Streptococcus pneumonia and Haemophilus influenza.