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Five years after the avian influenza strain H5N1 started killing poultry and people in Southeast Asia, researchers still don't know what to make of the dangerous and unpredictable virus.

After cutting an ever-widening swath through poultry flocks and infecting – and killing – mounting numbers of people in Asia, the Middle East and Africa, the virus seemed almost to take a breather in 2008. The year that just ended saw fewer recorded human cases than any since 2003, when this cycle of H5N1 activity began. It raises the question: Is H5N1 on the wane?

Sadly, science knows too little about how flu viruses emerge, spread and jump – or don't jump – from one species to the next to answer that question. Given the knowledge gap, scientists are still pushing for pandemic preparedness.

“Whether or not H5N1 virus is going to cause a human pandemic – nobody can predict that,” says Tim Uyeki, deputy chief of influenza surveillance and prevention for the U.S. Centers for Disease Control.

“I believe it's still a threat. But it's not the only threat,” he adds, noting a two-month-old Chinese girl was hospitalized in Hong Kong last month with an H9N2 avian flu infection.

Whatever the uncertainty about H5N1, one thing is clear. A fog of exhaustion has settled over the influenza science community as well as the public health officials who have been slaving over pandemic plans. A healthy portion of the broader public is probably sick to death of the subject, too.

“I think flu fatigue is certainly a phrase which is thrown around a fair amount in the past several months or past year or so,” admits Keiji Fukuda, head of the World Health Organization's global influenza program. “In stepping back after going through three or four years of working really hard, I think that there is a genuine sense of ‘Wow! We have been pushing so hard and we are tired of that.'” After a peak of 115 human cases and 79 deaths in nine countries in 2006, human infections declined to 88 cases and 59 deaths in 2007. In 2008, only 40 human cases and 30 deaths were reported, from six countries.

Experts can only guess at why that is. “It could be just a cyclical thing,” says Maria Zambon, head of the respiratory viruses unit of Britain's Health Protection Agency. “I would be cautious, I think about … inferring long-term trends from actually reasonably limited data.”

Dr. Fukuda says studies haven't shown that the virus has fundamentally changed, so the best guess is that the reason for the decline in cases probably rests with human behaviour. Efforts to eradicate infected poultry have improved, and countries that use poultry vaccine may be lowering the number of times people come in contact with the viruses as a result.

Dr. Uyeki suggests other possibilities. As the problem has seemed to wane, so has attention on it. That could be translating into more lax surveillance for new cases. Doctors could be less likely to suspect and test for H5N1 infection, attributing illness to myriad other potential causes.

It's clear the threat of a flu pandemic – caused by H5N1 or H9N2 or one of a multitude of other influenza viruses – remains. But it's also clear there are a vast number of other health problems competing for funding in highly challenging economic times.

It is a concern, Dr. Fukuda admits. “If we begin to withdraw our attention and move our attention to something else which is completely different, then we really stand to lose a lot of the work which has been built up over the past four years. To do this over and over again is truly ... it's like being Sisyphus,” he says, referring to the Greek myth of a man condemned to a task that can never be completed.