Epidemiology Influenza is a seasonal disease in temperate regions. Most cases in Australia occur during the winter months between June and September.9 In the northern hemisphere, influenza usually occurs between December and the next April, whereas in the tropics, flu can occur all year round. The number of affected people varies considerably from year to year. Epidemics of influenza occur during winter almost every year, more often due to a single virus type. However, different influenza virus strains may appear sequentially or simultaneously, and they can vary with geographic location.10 In Australia, there are, on average, dozens of deaths and thousands of hospitalisations per year specifically recorded as being due to influenza. However, these figures are underestimates, as a substantial proportion of influenza disease is not recognised and/or recorded by these means.5 For example, if a person who has influenza and then develops pneumonia dies, the cause of death may just be given as bacterial pneumonia. A study using mathematical modelling estimated that influenza is actually associated with more than 13,000 hospitalisations and 3,000 deaths each year in Australia, in people over 50 years alone.
http://www.ncirs.edu.au/immunisation/fact-sheets/influenza-fact-sheet.pdf
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