Workers from Mexico City's General Hospital wait to be vaccinated during a swine flu outbreak that has killed at least 16 people. (Miguel Tovar/Associated Press)If you lived in Toronto during the height of the 2003 SARS outbreak, you probably:

• Saw lots of people wearing protective masks.
• Bought or thought about buying masks for your family.

Since that outbreak of severe acute respiratory syndrome, Canada has been a world leader in preparing for the next pandemic — whether it's a new strain of flu or some other infectious disease not yet on the World Health Organization's (WHO) radar.

The WHO believes that the world is currently closer to another influenza pandemic than it has been any time since 1968, when the last of the 20th century's three pandemics swept the globe. It has developed a global influenza preparedness plan outlining the role of the WHO and recommendations for national measures before and during pandemics.

The organization uses six phases to categorize the risks facing the globe.

Phase one is low risk of human cases. Phase six is "pandemic" — efficient and sustained human-to-human transmission of disease.

Phase three — the one the WHO says the world has been in for several years — is classified as "pandemic alert with no or limited human-to-human transmission." We're there because a new influenza virus subtype — H5N1 or bird flu — is causing disease in humans, but is not yet spreading efficiently and sustainably among humans.

On April 27, the WHO raised its pandemic alert level to phase four for the first time. It also revised its category descriptions, making it clear that a phase four declaration means human to human spread of the virus in one country and a significantly increased risk of pandemic. But a pandemic is not a foregone conclusion.

A phase five declaration would be a strong signal that a pandemic is imminent:

According to the WHO's website, phase six "is characterized by community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is under way."

WHO spokesman Gregory Hartl said U.S. health officials have taken the first steps toward producing a vaccine against the virus, but that it would take five or six months for it to be ready.

The Public Health Agency of Canada follows the WHO's categories — but has expanded them somewhat.

A nurse wears the protective clothing used when treating SARS patients outside the door of a quarantined patient in Toronto on March 17, 2003. (Kevin Frayer/Canadian Press)Dr. Allison McGeer, the director of infection control at Mount Sinai Hospital in Toronto, told CBC News that before SARS, the Mexican outbreak likely would not have been on the radar screens at all.

"When any jurisdiction now has an outbreak of any significance in their population, they report it immediately. Before SARS Mexico probably wouldn't have reported it — not because they were deliberately trying to be difficult, but because it didn't occur to us how quickly these viruses can travel."

The downside of getting the word out quickly, McGeer said, is that you're reporting before your investigation is complete: "You have a lot of lab tests pending and we all have to wait together to see what's going on."

What happens next in any outbreak that spreads to Canada depends on what scientists determine they're dealing with.

'If it's influenza we're not going to quarantine. We can't stop the spread'—Dr. Allison McGeer

During SARS, infected people were quarantined in an attempt to keep the disease from spreading. A provincial public health emergency was declared, restricting hospital-based services to urgent cases, limiting access by visitors, and mandating the use of protective equipment such as masks for health-care workers.

There was no vaccine to protect the uninfected. When it comes to flu, there are vaccines.

"If it's influenza we're not going to quarantine," McGeer said. "We can't stop the spread. We'll put programs in place to reduce the spread as much as possible and try to make sure we have our care organized in the best way."

In the United States, the Centers for Disease Control and Prevention (CDC) identifies medical emergencies and gets the word out on how people should react.

The CDC — like the Public Health Agency of Canada — doesn't necessarily wait for the WHO to declare an emergency. The WHO set up the Centre for Strategic Health Operations in 2004 to provide a single point of co-ordination for response to acute public health crises including infectious disease outbreaks, natural disasters and chemical emergencies. Its first major task was to assist with emergency co-ordination during the tsunami that hit south Asia on Dec. 26, 2004.

Among the facilities the WHO relies on for its information is PHAC's national microbiology lab in Winnipeg. It is one of 15 laboratories around the world designed to accommodate the most basic to the most deadly infectious organisms. The lab tested samples from the Mexican outbreak and confirmed that it was a new mutation of swine flu.

Dr. Michael Gardham of the Ontario Agency for Health Protection and Promotion, says influenza is very different from SARS.

"We've been preparing for years for influenza … A virus like this may spread more widely than your usual flu virus. It doesn't however necessarily mean it's more dangerous than your regular flu virus. Flu viruses mutate all the time."