Bird flu is caused by a type of influenza virus that rarely infects humans. But when bird flu does strike humans, it's often deadly. More than half the people who become infected with bird flu die of the disease.
In recent years, outbreaks of bird flu have occurred in Asia, Africa and parts of Europe. Most people who have developed symptoms of bird flu have had close contact with sick birds. In a few cases, bird flu has passed from one person to another.
Health officials worry that a global outbreak could occur if a bird flu virus mutates into a form that transmits more easily from person to person. Researchers are working on vaccines to help protect people from bird flu.
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