Australian Health Management Plan for Pandemic Influenza

A1. What is influenza

Influenza (flu) is a potentially life threatening illness. It is a contagious disease (see Box 3) of the respiratory tract caused by influenza viruses. Each year, influenza causes serious infection and death around the globe, usually in the winter months (seasonal influenza).

Box 3: How is influenza spread The main ways in which the influenza virus can enter a person’s body and cause disease are via: Respiratory spread—when an infected person exhales, their respiratory droplets can spread into the eyes, nose and mouth of an uninfected person. The uninfected person needs to be relatively close by—usually at a distance of around a metre. Contact spread—if an uninfected person has virus on their hands and they touch their own eyes, nose or mouth they can infect themselves. A person’s hands may be contaminated by: touching used tissues, doorknobs or other items or surfaces that an infectious person has contaminated, virus present in faeces, blood or other bodily fluids. As the virus can not survive well in the environment, contamination of hands is most likely to occur following contact with freshly soiled items. Respiratory droplet and contact spread are the major modes of transmission in the community. Specific procedures within the medical setting may lead to generation of aerosols, requiring specific precautions in these settings.

Due to some pre-existing immunity to the seasonal strains of influenza, most people only suffer a self-limiting illness, lasting from a few days to several weeks. Influenza can lead to complications (see Box 4) and for some people—the elderly, people with poor immune systems and people with pre-existing respiratory, cardiac and endocrine disease—influenza can be a significant disease and cause death. It can also cause the death of healthy adults and children.

Box 4: Complications of influenza infection: difficulty breathing secondary bacterial infections in the lungs (pneumonia), the middle ear (otitis media) and sinuses (sinusitis) viral pneumonia inflammation of muscles (myositis), occasionally including heart muscle (myocarditis) neurological problems kidney failure heart attack stroke complications of diabetes.

Three different types of influenza viruses infect humans—types A, B and C. Only influenza A and B cause major outbreaks and severe disease. They are included in seasonal influenza vaccines. Influenza C causes a common cold-like illness in children. Only influenza A is known to have been responsible for influenza pandemics.

Influenza A and B viruses have two main proteins on the outside of the virus: the haemagglutinin (H); and the neuraminidase (N) proteins. These proteins are referred to as ‘antigens’ because they are the structures to which our immune system responds. New strains of influenza A and B continually emerge because of the tendency of these H and N antigens to change.

While influenza B is essentially a human disease, influenza A viruses are found in many species. Influenza A occurs as distinct forms or subtypes based principally on their haemagglutinin antigen, of which there are 16 different versions. Water birds are the natural host of influenza A viruses. From time to time influenza A viruses have, however, successfully jumped the species barrier and have become established in other animals, including humans.

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This information is current for 03 September, 2010
This information was issued on 05 December, 2008