Types of influenza

Page last updated: 23 February 2015

Seasonal influenza

Influenza, or 'the flu', is a viral infection of the nose, throat and lungs (the respiratory system). In Australia it usually affects people during the winter months from June to September. The flu viruses that circulate every winter are often similar to those from the preceding winter, so there is already a level of immunity (body defences) in the community. Seasonal flu most commonly affects the very young or the elderly.

Bird flu

Avian influenza - also known as 'bird flu' - is used to describe influenza virus A subtypes that primarily affect birds. Strains that cause a high proportion of deaths in affected flocks are called 'highly pathogenic avian influenza'. One of these strains is H5N1.

The H5N1 strain of bird flu is presently causing disease in water birds and domestic poultry such as chickens, geese and ducks in many countries across Asia, Europe and Africa. While there is no evidence that the H5N1 strain of bird flu is in Australia, it is possible that H5N1 could arrive in migratory birds. The risk of H5N1 infecting Australian poultry flocks, however, is considered low. Should an outbreak of bird flu occur, Australia has well-tested plans to contain and eradicate the disease. For more information see the AusVet Plan.

The H5N1 strain was recognised as a potential human health issue in Hong Kong in 1997, where it caused outbreaks in poultry and led to infections in 18 people, with 6 deaths. Fortunately, the outbreak was halted in Hong Kong by strict control measures in the poultry industry.

It is currently very difficult for the H5N1 virus to be transmitted from birds to humans - it requires very close contact with sick or dead birds. Although many thousands of individuals are likely to have been exposed to the virus during outbreaks in poultry of the disease, there have been a small but regular number of human infection cases recorded each year since 2003. This shows that the H5N1 virus is not, at this stage, well adapted to causing infection in humans. However, in cases where it has been transmitted, it has caused severe illness and the death rate has been high. Human cases have almost all been in people who had close contact with infected poultry, usually from their own farms.

The World Health Organization website provides the latest updates, news and answers to frequently asked questions about human cases of bird flu.

If you are planning overseas travel, see the Australian Government's travel advisory service to find out if the country you intend travelling to is affected by bird flu.

Pandemic (H1N1) Influenza 2009 (Swine flu)

Swine flu virus is derived predominantly from strains that affect pigs. A new H1N1 influenza virus derived from human, swine and avian strains was initially reported in April 2009 in Mexico and subsequently spread around the world. Cases of pandemic (H1N1) influenza 2009 have been confirmed in most other countries throughout the world by the World Health Organization.

The pandemic (H1N1) influenza 2009 virus is mild in most people. However, in a small proportion of people the virus causes death due to viral pneumonia and lung failure. High risk groups have been identified where the illness is more likely to cause complications, including patients with chronic respiratory conditions, pregnant women, patients who are obese (BMI >30), indigenous people and patients with chronic cardiac, neurological and immune conditions. Children and younger people have also been shown to be at increased risk of serious complications as well as rapid spreaders of the virus. Thirty percent of deaths have occurred in previously healthy people.

In Australia during 2009, there were 37,636 cases of pandemic (H1N1) 2009 influenza infection, including 191 associated deaths. The median age of those dying was 53 years, compared to 83 years for seasonal influenza.

For further information refer to the Pandemic (H1N1) Influenza 2009 page.