The virus behind the global influenza outbreak may be known as swine flu, but it didn't just come 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 world. To date, the pandemic has caused at least 3,205 deaths, as of Sept. 6, according to the World Health Organization.

As it resurges in the U.S. during flu season, health officials fear it could spread to half the population and are calling for urgent preparations, including the inoculation of children, pregnant women and other vulnerable groups when a vaccine becomes available in October.

Here's what researchers know about the virus so far:

H1N1's LINEAGE?

The new H1N1 strain is based primarily on an unusual virus that has been circulating widely in U.S. pigs since the 1990s. That "triple reassortant" flu is actually a combination of classic 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.

HOW DID STRAINS 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.

H1N1s BEFORE?

Yes. The extremely deadly 1918 Spanish flu was an H1N1 strain, and one of the strains of the seasonal flu is also an H1N1. But not all H1s and N1s are the same.

The H1 and N1 in the seasonal flu are both from humans. But new H1N1 is more virulent because most people never encountered it before, so they have no pre-existing immunity.

DEADLIER?

It doesn't seem to be deadlier than the seasonal flu, but it's hard to say for sure. Public health officials keep track of how many people have died from H1N1, but without reliable figures on total infections they don't know for sure what proportion of cases result in death.

Even if H1N1 is no deadlier than the seasonal flu, it will cause more deaths because it likely will infect more people.

LIKE THE 1918 FLU?

Both viruses arose in late spring -- the tail end of the traditional flu season. And both appear to be most dangerous for healthy people in the prime of their lives instead of the very young and very old.

The 1918 flu is thought to have begun with a springtime wave that was followed by a more lethal wave in the fall. It ultimately killed about 50 million worldwide.

NOT AS BAD THIS TIME?

Studies in ferrets found that the new H1N1 strain didn't mix with the seasonal flu when animals were infected with multiple viruses. That suggests it's not likely to mutate in a way that could make it more dangerous than it is now.

karen.kaplan@latimes.com

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