A pandemic is an infectious disease that spreads through a large part of the population over a wide area (for example a continent or even through the world). A widespread disease that is constant in terms of how many people are infected is not a pandemic. An illness that is not contagious is not a pandemic, even if many people are infected (for example, many people are affected by cancer but it is not contagious and hence it is not a pandemic).
The World Health Organisation has identified a number of steps by which influenza develops to become a pandemic. It typically starts with a virus that primarily affects animals. Then a few people become infected as a result of contact with the animals. The virus moves on to be spread between people and then becomes widespread and hence of pandemic proportions.
Recent examples of an influenza pandemic include swine flu and bird flu.
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