Quick Guide:Bird flu
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Global threat

Evolving menace

Virus origins

Human dimension

Fighting the virus

Human dimension
Members of a Turkish family died after exposure to sick birds

Although the present strain of bird flu does not cross species easily, it can pass to humans.

About half those who have been infected by H5N1 usually after close contact with birds have died.

But scientists say that any mutation may affect millions.

Symptoms range from fever, sore throat and cough to respiratory illness and organ failure.