For the first time in four decades the World Health Organisation (WHO) has declared a global flu pandemic.

One of the factors affecting its decision was the big increase in the number of swine flu cases in Australia.

There are now nearly 30,000 cases of swine flu worldwide and 141 people have died from the virus.

In the past week the number of cases in Australia has tripled to over 1,300.

Hong Kong is the latest to fall victim to the swine flu; 12 students at one school contracted the virus and now authorities have shut down every primary school for two weeks - 500,000 students have been told to stay at home.

It is a sign of just how worried some countries are about the spread of the swine flu.

The H1N1 virus started in Mexico in April and it has slowly spread to 74 countries.

But it has been the sharp rise in cases due to the Southern Hemisphere winter, particularly in Australia and in Chile, that triggered the emergency meeting of the WHO in Geneva over night.

WHO chief Margaret Chan emerged with some sobering news.

"The world is now at the start of the 2009 influenza pandemic. Countries where outbreaks appear to have peaked should prepare for a second wave of infection," she said.

"We are all in this together and we will all get through this together."

The pandemic alert has been raised to phase 6 - the highest level.

It is the first flu pandemic in 40 years. The last, the Hong Kong flu in 1968, killed about 1 million people worldwide.

This time the WHO does not expect to see a sudden and dramatic jump in the number of severe and fatal infections.

"The phase 6 doesn't mean anything concerning severity," WHO spokesman Gregory Hartl said.

"It is concerning geographic spread so we have to differentiate between the two.

"Pandemic means global but it doesn't have any connotation of severity or mildness."

The director of the Centre for Disease Control and Prevention in the United States, Thomas Frieden, says the WHO declaration is no surprise.

"It is expected based on the data. WHO waited until they were certain that they had documentation that in multiple continents there was person to person sustained transmission," he said.

"The declaration of a pandemic does not suggest that there has been any change in the behaviour of the virus, only that it is spreading in more parts of the world."

The WHO says the virus is not targeting older people, but rather the young under the age of 25 are predominantly falling ill with the disease.

Dr Mike Skinner is a virologist at the Imperial College London and he says older people appear to be immune.

"But what does seem to be protected are people over 60. It looks as though they have experienced a related virus back in the 50s or before and so this is why we are not seeing so many people over 60 being affected," he said.

Pharmaceutical companies have told the WHO that a vaccine will be available in a few months but that will not be until the end of the Australian winter.