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A pandemic is a global infectious disease outbreak. The term ‘pandemic’ relates to how the disease spreads, not how widespread it is or how many deaths occur (eg. cancer is widespread and kills many people but is not a pandemic).
From studies done by the WHO, pandemic influenza develops in three stages (also see Figure 3):
Stage 1: An influenza virus in an animal develops the ability to infect humans and cause serious disease. During this phase, the virus is not able to transmit efficiently between humans. Contact with infected animals is needed for human infection to occur.
Stage 2: Following a genetic change, the virus becomes more efficient at passing from human to human, first within small groups (eg. families or community networks) and later over wider but still localised areas.
Stage 3: Finally, the virus is able to transmit readily between humans. It spreads rapidly due to a short incubation period, period of communicability and the infectious nature of influenza. Rapid global spread is aided by extensive international travel, which takes place every day between virtually every country in the world.
Novel influenza viruses such as avian influenza (H5N1) continue to circulate globally and are a potential source for a pandemic.
Figure 3. Antigenic shift
Reproduced with permission from the National Institute of Allergy and Infectious Diseases. Links Studio.