The 10 Genes of a Human Flu Virus, Furiously Evolving

A quarter of birds typically carry two or more strains of flu at the same time, allowing the viruses to mix their genes into a genetic blur. "Birds are constantly mixing up the constellation of these viruses," said David Spiro of the J. Craig Venter Institute.

From birds, flu viruses have moved to animals, including pigs, horses and humans. Other viruses, like H.I.V. and SARS, have also managed to jump into our species, but many others have failed. "It's a very rare event when a virus creates a new epidemic in another species," said Colin Parrish of Cornell University. In Southeast Asia, for example, a strain of bird flu has killed hundreds of people in recent years, but it cannot seem to move easily from human to human.

Influenza viruses also moved from bird guts to human airways. That shift also required flu viruses to spread in a new way: in the droplets we release in our coughs and sneezes.

Dr. Spiro and his colleagues have also discovered that human flu viruses experience a lot of reassortment each season. In later years, some of the descendants of that strain picked up genes from bird flu viruses.

Reassortment also played a big role in the emergence of the current swine flu. Its genes come from several ancestors, which mainly infected pigs.


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