A colourized transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold) are shown in this 1997 image. THE CANADIAN PRESS/HO, CDC - Cynthia Goldsmith, Jackie Katz, Sharif Zaki

TORONTO - Canada has reported the first case in North America of an infection with the H5N1 avian flu virus.

A woman from Alberta who had travelled to Beijing, China in December fell ill and died after her return.

With H1N1 — seasonal flu — making headlines these days, this new development may trigger flu confusion. So here are some essential flu facts:

What is H5N1? It's the original "bird flu." This is the virus that burst out in Asia in late 2003 and early 2004. Millions of poultry died or were culled as the virus — which is highly infectious among birds — spread in Vietnam, Thailand and other countries. It hasn't been making as many headlines lately, but it's still causing bird outbreaks and occasional human cases in parts of Asia.

Highly infectious among birds? What about people? Since late 2003, just under 650 people from 15 — now 16 — countries are known to have contracted this strain of flu. But it rarely infects humans. Untold numbers of people in affected countries would have been exposed to it over the years but very few have gotten sick. And while there have been a few cases where one sick person spread it to others, those chains of transmission have always died out. Unlike human flu viruses, this virus is not an effective person-to-person spreader.

Is it likely the Edmonton case will lead to more infections? Health authorities say there are no signs of illness among of the woman's contacts or the health-care workers who cared for her. They'll need to watch those people for a couple of weeks to make sure. But this could be a one-off case.

So what's the fuss about? The virus doesn't spread person to person now. But science can't tell if it will evolve and acquire the capacity to spread human to human. So flu experts and the World Health Organization watch this virus closely. Also, about 60 per cent of people who have been known to have been infected have died. So while infections are rare, they're often severe.

Does a flu shot protect against H5N1? No. The seasonal flu vaccine protects against the human flu viruses that circulate every winter. Some manufacturers have made experimental H5N1 vaccines for testing purposes, but there is no H5N1 vaccine available for the public at this moment.

Is H5N1 related to H1N1, the virus we're hearing so much about this winter? All flu viruses have the same original source; they come from wild water birds like ducks. But some have found their way into animals — pigs, horses, dogs — and spread among them. And some have become human flu viruses: H1N1, H3N2 and influenza B viruses.

Human viruses? Bird viruses? What's the difference? Our immune systems have some experience with the seasonal viruses. From childhood, we've been infected sporadically throughout our lives. But animal or bird flu viruses look different enough genetically that our immune systems don't mount the kinds of response they do for regular flu. More people would be susceptible to them, so if they start to spread among people they can set off a huge wave of illness, called a pandemic. That happened in 2009 when a swine influenza virus, H1N1, started infecting people.

Will H5N1 cause a pandemic? Currently there is no way to know if it will or it won't.

News from © The Canadian Press, 2014