Pandemic influenza occurs periodically throughout history, causing widespread illness and death, overwhelming medical systems and wreaking chaos in societies. These viruses are highly contagious, and because they are new, no one is immune.

Influenza is serious enough, killing an average of 36,000 people in the USA every year, but because it is caused by strains of the virus that are known to be circulating in the world, vaccines can be prepared to prevent it. This year, a strain that doesn't match up exactly with those in the vaccine has emerged and raised serious concerns, but experts believe the vaccine will still offer some protection.

Using current technologies, it takes as long as six months to create flu vaccines.

"The world will be in deep trouble if the impending influenza pandemic strikes this week, this month, or even this year," write international flu experts Richard Webby and Robert Webster of St. Jude Children's Research Hospital in Memphis.

In an article in Friday's Science, they argue that new methods to produce flu vaccines rapidly are known, but have not yet been tested. "What's necessary is to do trial runs and demonstrate these (new vaccines) are safe," Webster said during a press briefing.

Also needed is a stockpile of anti-viral drugs that can be used to treat or prevent the spread of flu. Current supplies would last only days in a pandemic, Webster said, "but no country has yet invested in stockpiling."

It was international flu surveillance that helped alert the world to SARS this past spring.

In addition to new vaccine technologies and anti-flu drugs, he says, "you need the best surveillance possible, particularly in places where you think a virus might emerge."

It has been more than 30 years since the last pandemic, and "we're overdue," Webster says.

"Flu keeps knocking on the door. We've seen enough incidences over the past three or four years to make us very alarmed."