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HOW SWINE FLU OUTBREAK EMERGED

Flu viruses mutate over time causing small changes to proteins on their surface called antigens. If the immune system has met a particular strain of the virus before, it is likely to have some immunity; but if the antigens are new to the immune system, it will be weakened.

SWINE FLU - THE BASICS

Symptoms usually similar to seasonal flu - but deaths recorded in Mexico

It is a new version of the H1N1 strain which caused the 1918 flu pandemic

Too early to say whether it will lead to a pandemic

Current treatments do work, but there is no vaccine

Good personal hygiene, such as washing hands, covering nose when sneezing advised

How dangerous is it?

1918: The Spanish flu pandemic remains the most devastating outbreak of modern times. Caused by a form of the H1N1 strain of flu, it is estimated that up to 40% of the world's population were infected, and more than 50 million people died, with young adults particularly badly affected