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This is the scenario that has some scientists worried: The two viruses meet - possibly in Asia, where bird flu is endemic - and combine into a new bug that is both highly contagious and lethal and can spread around the world.
Scientists are unsure how likely this possibility is, but note that the new swine flu strain - a never-before-seen mixture of pig, human and bird viruses - has shown itself to be especially adept at snatching evolutionarily advantageous genetic material from other flu viruses.
Bird flu kills more than 60 percent of its human victims, but doesn't easily pass from person to person. Swine flu can be spread with a sneeze or handshake, but kills only a small fraction of the people it infects.