The word pandemic, from the Greek pan (= all) and demos (= people), means something that affects a whole people or everyone without exception. This is why it is not used for animal diseases, any more than the term epidemic (pandemic-like animal diseases are instead often described as panzootics). Unlike an epidemic, a pandemic is not confined to a particular region.

Flu pandemics occur, on average, every 40 years. For experts, it’s not a matter of whether, but when the next pandemic will occur. They consider that its likeliest cause will be the bird flu strain H5N1, first identified in the Far East

Bird flu and pandemics
Most bird flu viruses are not infectious to humans. However, if simultaneous infection with a bird and a human flu virus occurs in humans or pigs, the viral strains may combine and mutate, producing a completely new virus potentially capable of spreading from animal to human and from human to human. As such types of virus would have a composition quite different from that of previous viruses, previous flu vaccinations would no longer protect humans. Experts believe that viral strains with mutations of this sort could trigger the next flu pandemic.

World Health Organisation
In its Pandemic Preparedness Plan, the WHO has recommended that individual countries should stockpile antiviral drugs effective against all strains of flu virus. The Pandemic Preparedness Plan and information about the national plans implemented in 15 countries can be found on the Internet.

Flu pandemics in history

• Spanish flu (1918–1920), with 500 million cases and 25 to 50 million deaths worldwide [1]
• Asian flu (1957), one million deaths
• Hong Kong flu (1968), 700,000 deaths
• Hong Kong flu (1997)