When U.S. President George W. Bush recently and repeatedly voiced concern over the threat of an influenza pandemic, an issue that has been fuelling high anxiety among public health authorities and flu experts for quite some time spilled all over the popular press.

And with last month's discovery of birds infected with the worrisome H5N1 avian influenza virus in Romania and Turkey, and an unrelated discovery of several Canadian birds with an H5 strain that has yet to be identified, it seems the term "bird flu" is on everyone's lips.

The heightened attention carries with it a lot of confusion. So The Canadian Press asked some experts on avian and human influenza for help explaining what these viruses are -- and are not.

The name game

First things first: It's catchy and it's easy to say, but influenza experts don't like and don't generally use the term "bird flu." At their origins, all influenza viruses probably came from certain species of wild waterfowl, so the term is too vague to be useful.

Recent developments in Colombia illustrate the problem.

An avian flu strain found there, an H9, bears no relation to the one plaguing Southeast Asia. It's a far milder strain that will cause economic problems for affected poultry farmers, but probably poses next-to-no risk to people at this point.

A rose is not a rose

Influenza A viruses are divided into categories or subtypes based on two genes they carry on their surface. To date, scientists have found 16 hemagluttinins and nine neuraminidases, the H and the N in a flu virus's name.

In theory there may be 144 different combinations, all of which could be called a "bird flu.'' But some Hs and Ns have never been found together, leading experts to hypothesize some combinations cannot be formed.

Most of the known combinations have shown no talent at infecting people.

"The vast, vast majority of these (avian) viruses wouldn't do anything in humans,'' says Michael Perdue, an avian influenza expert with the World Health Organization.

According to an article Perdue and co-author Dr. David Swayne published in Avian Diseases in July, human infections have been documented with only H5N1, a few H7s -- H7N2, H7N2 and H7N7 -- and H9N2 avian flu viruses.

H7N3 was behind the large poultry outbreak in British Columbia in 2004. Two people were infected, but they suffered only conjunctivitis and mild flu-like symptoms. One person died in an extensive H7N7 outbreak in the Netherlands in 2003, but the majority of the nearly 90 documented human cases there suffered mild symptoms too.

The Asian version of H5N1 is far and away the worst known avian flu virus when it triggers human infections. The official WHO count suggests there have been 137 human cases since 1997, when it first jumped into humans. Of those known cases, 67 have died.

Public understanding of the vast differences between the various subtypes is low, suggests Swayne, director of the U.S. Department of Agriculture's Southeast Poultry Research Laboratory in Athens, Ga.

"When they say: 'I'm scared this bird flu's going to kill me.' Well, it's like: 'Wait a minute. Here's a low path, economic issue of avian influenza in a chicken flock. It really has no risk for humans or very minimal risk for humans and it's not the same as the Asian H5N1.'"

High path, low path

The term "low path" Swayne uses describes another important distinction between avian flu viruses. Most are what is known as low pathogenicity - low path for short. When they jump into domestic poultry, they don't even kill chickens. Typically egg production drops off.

Only the H5 and H7 subtypes produce viruses that are high pathogenicity or "high path," explains Dr. Richard Slemons, an avian influenza expert at Ohio State University.

High path viruses wipe out poultry flocks. The Asian H5N1 everyone is so worried about is a high path virus.

But not all H5s and H7s are high path. Mexico has had a lingering low path H5N2 outbreak among poultry flocks for more than a decade.

Not all H5N1s are created equal

Even within a subtype, there is a lot of variation among viruses. There is no cookie cutter that punches out identical H5N1s.

"What's happened is we've taken this term, this term H5N1, and we have equated it to being for all viruses that have that terminology as being the same as the Asian virus. And it's not," Swayne says.

He likens it to looking through a phone directory and assuming that all the Joneses listed are related.

So when H5N1 viruses have been found in Russia, Turkey and Romania, scientists needed to perform genetic analysis on them to see how closely matched they are to the Asian viruses.

Unfortunately, they have been found to be closely related to that lethal virus. But they might not have been. For instance, Slemons found a low path H5N1 in a mallard in Ohio in 1986. It would not have posed a fraction of the threat to human health that the Asian H5N1 does.

Economics vs. Public Health

In fact, most avian flu viruses are a much great economic than human health risk.

These viruses don't cross into people very often. In their paper in Avian Diseases, Perdue and Swayne documented only 234 cases dating back to 1959 where people were shown to have been infected by avian viruses. (That figure was as of June 28; there have been additional H5N1 cases in Indonesia, Thailand and Vietnam since then.)

"The question might be: Out of all the billions of people that have been exposed, why so few have been infected?" Slemons noted.

Science doesn't have the answer to that question at present.

But things are more clear cut on the economic side of the equation. Outbreaks can be costly and difficult to halt. At one point, farmers in British Columbia estimated it would cost $340 million to rebuild their battered industry after their H7N3 outbreak.

And if the outbreak is caused by a high path strain, poultry exports of an affected nation can find international doors slam shut on their products.

Where the threat lies

Still, the threat remains that an avian virus could mutate and start spreading easily from person to person. Because human immune systems have no antibodies to those viruses, that would trigger a pandemic. Just how bad a pandemic it would be would depend on how virulent the resulting virus was.

So the situation with H5N1 is a source of ongoing and significant concern. And while no one likes to see the strain increase its geographic reach, experts believe the threat remains greatest in Asia, where high concentrations of people, birds and virus could fuel the emergence of mutations that facilitate human-to-human spread.

"The casino for genetic roulette will still be in Asia," predicts Dr. Michael Osterholm, director of the Center for Infectious Disease Policy and Research at the University of Minnesota.

"It's not that it couldn't appear there," he says, referring to Europe. "But probability favours the drive towards the ... mutations resulting in a human-transmitted agent in Asia."