Introduction

Pandemic influenza is very different from, and far more serious than, the usual seasonal influenza outbreaks that happen every year. A pandemic is an outbreak that may affect hundreds of millions of people worldwide, leading to a large proportion of them becoming ill. Pandemic flu occurs when a new influenza virus emerges that is very different from ordinary seasonal flu and, because it is new, people have very little or no immunity to it. There are normally around two to three such pandemics every century. The most recent pandemic was in 1968 and affected mainly the young and the elderly. An earlier outbreak in 1918 caused an estimated 20-40 million deaths worldwide, most of which occurred among people aged between 20 and 45.

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