Menu Close

Explainer: what is the flu?

Influenza symptoms include high fevers and chills, as well as pains and aches in muscles and joints. Davide Taviani

When people say they have “the flu”, what they’re experiencing most of the time is the common cold, which is not caused by the influenza virus at all.

But the term “flu season”, which Australia is in the midst of right now, is correct because influenza-related illness is seasonal. It’s more common in winter although the viruses circulate all year round, particularly in tropical areas.

Influenza is a tricky virus. It changes the proteins that make up its outer coating frequently. The two important ones that scientists use to name various strains are called haemagglutanin (H) and neuraminidase (N). Different combinations of these two make up the strains of influenza that we hear about – H1N1, H5N2 and so on.

Cold or flu?

The common cold features mainly upper respiratory symptoms - runny nose, sore throat and coughs. Although people often feel a bit “sick” (temperature, aches and pains and tiredness), this is not a major feature of the illness, and it settles within a few days.

In influenza, these “systemic symptoms” are much more prominent - high fevers (38° to 39° Celsius) that are sometimes accompanied by chills (uncontrollable shaking). Pains and aches in muscles and joints often confine people to bed. The cold symptoms mentioned above may also occur, but you’ll usually be too unwell to be bothered by them.

Flu comes on very suddenly and the fevers settle after a few days. But complete recovery often takes one or two weeks. So, although it is an unpleasant illness, most people with influenza will recover with time.

Pneumonia (inflammation of the lungs) is the most common complication of the flu. It may be caused by the influenza virus itself, or secondary infection from a bacteria (such as Staphylococcus aureus).

The very young and very old are most at risk of complicated influenza, as well as people with chronic lung disease (asthma or emphysema), those whose immunity is not at its best (immunosuppression from cancer, HIV or medications), the very obese and pregnant women.

A map showing the seasonal risk areas for influenza: blue is November to April; yellow is all year round; red is April to November; and grey marks the areas where there’s no significant flu virus population. Wikimedia Commons


Preventing illness is always more effective than treatment. As well as not going to work when you’re unwell (and encouraging your co-workers to do the same), coughing into your sleeve (or a tissue) and washing your hands frequently are simple but important steps that everyone can take to protect themselves.

The flu vaccine is thought to be around two-thirds effective in preventing infection, although, in the real world, this figure may seem lower because not every “influenza-like illness” is actually caused by the flu virus.

The vaccines available in Australia don’t contain any live virus, so the flu vaccine does not give you the flu. What it does is cause your body to mount an immune response to the vaccination, which can cause some symptoms for a couple of days.

This might make you feel a bit unwell for a couple of days, but it’s definitely better than actually getting the flu.


Australia’s national immunisation program recommends (and pays for) vaccination for the following groups:

  • all people aged over 65;
  • Indigenous Australians aged over 15;
  • all pregnant women; and
  • anyone older than six months with a chronic heart or lung condition, severe asthma or diabetes.

The vaccine is safe in pregnancy - and much safer than getting the flu while you’re pregnant. It is only not recommended for people with an allergic reaction to a previous vaccine.

The flu vaccine available in Australia is grown using hen’s eggs but people with an egg allergy can probably still be vaccinated and should ask their doctor for advice.

One preparation of the vaccine against 2009 H1N1 influenza virus was associated with a greater risk of febrile convulsions when given to children under the age of five, but other vaccines don’t pose the same risk.

The problem with the flu vaccine is really more a problem with the virus. Different strains may circulate each year, and the virus changes so quickly that last year’s vaccine may not provide full cover, even if the strains are the same. This means a vaccination is needed every year.

The flu vaccine is not subsidised for people who are not in a risk group (see above), although it’s still encouraged, as more vaccinated people will reduce transmission for everyone.

Flu vaccination is not mandatory - even for health-care workers (although some people think it should be).

In my experience, most people who have had true influenza are at the front of next year’s queue for vaccination.

Want to write?

Write an article and join a growing community of more than 182,400 academics and researchers from 4,942 institutions.

Register now