What causes the flu?

There are three types of influenza viruses: types A, B and C.

Influenza A and B generally cause indistinguishable diseases, although illness due to influenza B can be milder than that caused by influenza A. Influenza C causes a mild respiratory infection in young children.

You may be infected with any combination of the viruses in the same season even, although rarely at the same time.

The notation of influenza viruses
Influenza A viruses have two kinds of protein spikes on the outermost layer (or envelope). These spikes (called haemagglutinin and neuraminidase) are distinguishing features of each virus, and represent the way in which influenza viruses are named.

A typical influenza strain might be named A/Moscow/10/99 (H3N2), where A represents the type of influenza, Moscow the place of origin, 10 the 10th virus isolated from the place of origin in the particular year, 99 the year it was isolated, and H3N2 the specific spikes present on the outside of the virus’s envelope.

There are 15 different haemagglutinin molecules of which only the first three regularly infect humans, and nine neuraminidase molecules, of which the first two infect humans. Only one of each type of spike can be present on any virus. Humans are occasionally infected by H5 influenza A (bird flu) viruses, but cannot pass these on to other humans.

How does the flu virus spread?

Step 1. An infected person sheds the virus from their nose and throat. This starts at about the same time as flu symptoms begin and continues for about a week.

Step 2. By coughing or sneezing the virus is then transmitted in one of three ways:

  • Step 2a. The virus can be spread in large droplets, regarded as the most common method of spreading.
  • Step 2b. It can also be spread by infectious aerosols, which are tiny particles in the air, small and light enough not to settle on surfaces. These aerosols can typically stay in suspension for up to 60 minutes.
  • Step 2c. The virus can contaminate hands, tissues, handkerchiefs or other objects after the affected person has sneezed or coughed. If someone else touches these objects, they may contract the virus.

Step 3. When the virus reaches the cells of your airways – either because of breathing in droplets or aerosols or because you’ve touched an infected surface – it can establish an infection and has thus successfully spread.

Reviewed by Cape Town-based general practitioner, Dr Dalia Hack. March 2019.

Read more:

- Flu risk factors

- Who gets the flu?

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