There's no question H5N1 bird flu has 'pandemic potential.' How likely is that worst-case scenario?

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As early as 1997, just a year after H5N1 was first discovered, there were warning signs this form of avian flu was capable of wreaking havoc far beyond birds.

That year, poultry outbreaks in China and Hong Kong were linked to 18 human infections — and a third of those people later died. 

In the decades that followed, alarm bells from the scientific community rang louder and louder. H5N1 began infecting dozens more species, from tens of millions of wild and farmed birds, to raccoons and foxes, to seals and sea lions to, most recently, U.S. dairy cattle. The virus eventually caused more than 800 human infections around the world with a stunning death rate — for known cases — of roughly 50 per cent.

"The increasing host range of the virus, [its] potential spread among mammals and between a mammal and a human, its wide geographical spread, and the unprecedented scale of the outbreaks in birds, raise concerns about the pandemic potential" of H5N1, warned a May editorial in leading medical journal The Lancet Infectious Diseases.

Given those factors, an avian flu pandemic may feel not just inevitable, but imminent. 

Yet the reality is murkier. Though influenza viruses have a knack for rapid evolution, this one hasn't managed to adapt to sustain human-to-human transmission, despite circulating for the last three decades.

"But that could still potentially happen," said epidemiologist Timothy Sly, a longtime influenza researcher and professor emeritus with Toronto Metropolitan University's school of public health.

"And if it ever did happen… it would be a global health catastrophe."

So how likely is that kind of worst-case scenario? And if H5N1 does start spreading person to person — in a month, or a year, or a decade — what would its impacts really be?

CBC News talked to several researchers, including those studying H5N1 now and others who've followed influenza trends for decades, to help unpack the crucial unanswered questions around this virus's capacity for sparking a possible pandemic.

"Are we watching the beginning? Is this where we're seeing repeated spillovers that one day take off? Or is it like in COVID, where suddenly there's a cluster of human cases and then it's too late?" questioned Louise Moncla, an avian flu researcher and assistant professor at the University of Pennsylvania.

There's no way to know, she says. 

That's why scientists like Moncla are watching the virus closely to understand its transmission patterns and ongoing evolution, all to spot early signals that a storm may be brewing.