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TORONTO - The World Health Organization's top flu scientist often describes the virus he's studied for years as "humbling."
And Dr. Keiji Fukuda isn't alone in marvelling at the mercurial nature of influenza. Flu scientists repeat almost as a mantra that the only thing predictable about flu is its unpredictability.
Yet despite decades of evidence that influenza will repeatedly rewrite the rules, flu dogma emerges and takes hold. Scientists keen to sift patterns from chaos agree X is true about Y - until the virus sets them straight yet again.
In the late '60s it was held that pandemic viruses emerged in 11-year cycles, after the closely spaced 1957 Asian flu and 1968 Hong Kong flu outbreaks.
It used to be accepted that only H1, H2 and H3 viruses could infect humans. And then viruses from the H5, H7 and H9 subtypes jumped from birds to infect people. Wrong again.
Though the world is not quite seven months into this pandemic, a number of widely held assumptions about flu and pandemics seem destined for the redrawing board when the dust from this outbreak settles.
Here are some:
-Pandemic viruses emerge from Asia, the cradle of flu viruses.
Years of focus on H5N1 avian influenza viruses left experts convinced Asia was the birthplace of new flu viruses and would be the source of the next pandemic. Despite the fact that there's good evidence the 1918 Spanish flu virus may have emerged in Kansas, no one was looking to North America as ground zero for the first pandemic of the 21st century.
It's a valuable lesson, says Dr. Nancy Cox, who has been pushing for a number of years for more flu surveillance in Latin America.
"You can't take your eye off the other possible threats. You can't focus too much on one area of the world because influenza - a new virus - can emerge from anywhere," says Cox, head of the influenza division at the U.S. Centers for Disease Control.
-Pandemics are triggered by "antigenic shift."
Flu viruses evolve constantly via small mutations, a process called antigenic drift. But once in a blue moon an entirely new virus bursts out of nature, an event known as antigenic shift. Because most people are vulnerable to the new virus, it ignites a pandemic.
It used to be thought pandemics could only be started by a virus with a new hemagglutinin - the H number in the virus's name - or a virus with a hemagglutinin that hadn't spread recently among people, such as the H2N2 viruses that circulated from 1957 to 1968.
The current pandemic is caused by an H1N1 virus, which is startling because almost everyone alive has antibodies to H1 viruses. They've been circulating among people since 1918, except for a 20-year gap between 1957 and 1977.
So few scientists would have predicted a new H1 virus could cause a pandemic at this point in history.
Some, in fact, still question whether this outbreak is a pandemic, at least by the definition science currently applies. The retired head of virology for the U.S. Centers for Disease Control is one of the doubters.
"There's no precedence for this," says Dr. Walter Dowdle, who now works for the non-profit Task Force on Global Health, based at Emory University in Atlanta. "Nobody had really thought that . . . the virus would re-emerge with this much background immunity."
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