The virus behind the current influenza pandemic may be known as swine flu, but it didn't come only from pigs. Wild birds and humans also played a role in its creation. Scientists are still trying to unravel how it wound up infecting people and spreading rapidly around the globe. Here's what they know so far.
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What's the lineage of H1N1?
The new H1N1 strain is based primarily on an unusual influenza virus that has been circulating widely in U.S. pigs since the mid-1990s. That "triple reassortant" flu is actually a combination of classical swine flu, a North American avian flu, and a strain of human flu. Somehow, a single pig became simultaneously infected with that virus and a pure swine flu strain found in pigs in Europe and Asia. The two strains swapped genetic material to produce the new H1N1 strain, which then began to infect humans.
By comparing the small genetic differences in flu samples from many patients, scientists estimate the pandemic strain was already circulating in people by November 2008. The earliest cases to be confirmed in people occurred in Mexico in March.
How did the two strains of swine flu mix?
That remains a mystery, and scientists will probably never know. Relatively few pigs engage in intercontinental travel, and those that do are strictly quarantined.
But there are theories. One is that a person in Asia became infected with the Eurasian swine flu, then traveled to North America and passed it along to a pig here that already had the triple reassortant virus. That would explain why the outbreak began in Mexico and the United States.
Another theory holds that an infected North American pig traveled to Asia and passed along its virus to another pig with the Eurasian flu strain. That pig then infected a person, who brought the virus back to North America and spread it to other people. This would explain why H1N1 has seven out of eight genes in common with a flu sample taken from a Hong Kong pig in 2004.
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Why is it so easy for flu strains to mix and match?
When a flu virus infects a cell, it breaks down into its eight component genes and invades the cell's nucleus. Once inside, those genes make hundreds of copies of themselves. Then they exit the nucleus and repackage themselves into new flu particles, which go on to infect additional cells. If a single cell is infected with two strains of flu at the same time -- which can happen easily -- genes from both can be bundled together to form a new virus.
