Mutations in virus lead to epidemics

Influenza A's ability to re-invent itself has been the main factor behind major epidemics.

The three main strains - A, B and C - have been around for centuries, but influenza A is the most dangerous threat because of its constant mutation.

Major outbreaks have included the pandemic of 1918-19 which claimed at least 20 million lives, with some historians putting the figure as high as 40 million.

The last epidemic seen in Hong Kong was in 1968 when 45 people died.

New strains pose a larger threat because scientists cannot keep up with the new virus in terms of producing a vaccine.

Influenza A changes continually in response to immunity in humans and animals, and is highly prevalent in aquatic birds and farm animals.