Sometime in the last few years, as the world's attention was focused on the bird flu that killed more than 250 people in Asia, another bird flu strain infected pigs.
It mixed with two kinds of flu that are endemic in swine and a fourth that originally came from people.
For The Record
Los Angeles Times Tuesday, April 28, 2009 Home Edition Main News Part A Page 4 National Desk 1 inches; 53 words Type of Material: Correction
Swine flu virus: In Monday's Section A, an article in Monday's Section A about the intricacies of the swine flu virus said that more than 250 people in Asia had died from the recent bird flu outbreak. The World Health Organization reported a worldwide total of 257 dead, 222 of them in Asia.
The resulting concoction spread among pigs, then recently -- no one yet knows where or when -- started infecting humans. Since late March, it seems to have sickened people in California, Texas, New York, Kansas and Ohio, as well as in Canada and Mexico, where there have been 22 confirmed deaths.
Scientists said Sunday that they were unable to predict what this new swine flu would do next.
"It's impossible to say with any assurance what's going to happen," said Dr. Christopher Olsen, a molecular virologist who studies swine flu at the University of Wisconsin School of Veterinary Medicine in Madison. "Influenza viruses can evolve quite quickly."
Inspecting the virus itself is of little help, because scientists have yet to identify which features help it spread or kill, said Dr. Scott Layne, an epidemiologist at the UCLA School of Public Health.
"The microscope doesn't tell you anything," Layne said. "What are the genetic correlates of virulence? Unknown. Transmissibility? Unknown."
--
The challenge of flu
Among threats to public health, influenza poses an unusual challenge. People, pigs, birds and horses have developed unique strains of flu, which can easily mix and match into new ones that the human immune system is ill-equipped to recognize.
And because the eight genes that form all Type A flu viruses -- the most dangerous kind -- are made of RNA instead of DNA, they don't copy themselves reliably and are prone to further mutation.
Flu research has accelerated since the Asian bird flu spread to humans in 1997. But the more scientists study it, the more questions they have.
"I know less about influenza today than I did 10 years ago," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Yet for all its destructive power, the influenza virus is a straightforward organism.
Its outer shell is studded with a protein called hemagglutinin that allows flu particles to attach to cells lining the respiratory tract. The virus then takes over the host cell and uses it to make hundreds of copies. Those new flu particles use another surface protein, neuraminidase, to break off from the host so they can search for new targets.
