DUBAI, 11 June 2009 (IRIN) - In the space of three months, the world has watched what seemed to be a localized flu outbreak in Mexico spread to 74 countries, infect some 27,737 people and be declared pandemic on 11 June by the World Health Organization (WHO).

"The world is moving into the early days of its first influenza pandemic in the 21st century," Margaret Chan, director-general of WHO, told reporters. "The virus is now unstoppable."

As of 10 June, 139 people had died from the A(H1N1) influenza virus - dubbed 'swine flu’ because of its apparent origin from pigs.

Such has been the recent proliferation of the virus that WHO declared an emergency meeting on 11 June to classify it a pandemic and take it to the sixth and last of its alert phases.

A disease is classified as pandemic when its transmission between humans becomes widespread in two regions of the world.

The WHO says “an influenza pandemic is caused by a virus that is either entirely new or has not circulated recently and widely in the human population. This creates an almost universal vulnerability to infection. While not all people ever become infected during a pandemic, nearly all people are susceptible to infection.”

This is the first time in 40 years that the world experiences a pandemic. The last was the Hong Kong flu in 1968 which killed about one million people.

The WHO emergency meeting in Geneva on 11 June was prompted by a four-fold increase in A(H1N1) cases in Australia in the past week - bringing its total cases to 1,224. In Hong Kong - where 50 cases have been confirmed, including 12 students - the territory’s chief executive said all primary schools and nurseries would be shut for two weeks beginning on Friday 12 June in a bid to curb the spread of the virus.

Countries with the most swine flu deaths/cases :

World: 27,737 cases – 141 deaths Mexico: 5,717 cases – 106 deaths USA: 13,217 cases – 27 deaths Canada: 2,446 cases – 4 deaths Chile: 1,694 cases – 2 deaths Costa Rica: 93 cases – 1 death Dominican Republic: 91 cases – 1 death Australia: 1,224 cases – 0 deaths United Kingdom: – 666 cases – 0 deaths Japan: 485 cases – 0 deaths Spain: 331 cases – 0 deaths Argentina: 235 cases – 0 deaths Panama: 221 cases – 0 deaths China: 142 cases – 0 deaths

Six phases

The WHO has six pandemic influenza phases. The grouping and description of these pandemic phases were revised on 27 April 2009 to “make them easier to understand, more precise, and based upon observable phenomena”.

Phases 1-3 are predominantly animal infections with very few human infections. These phases correlate with preparedness, including capacity development and response planning activities.

In Phase 4 there is sustained human transmission. According to WHO, “Phase 4 indicates a significant increase in risk of a pandemic but does not necessarily mean that a pandemic is a foregone conclusion.”

Phase 5 is characterized by the human-to-human spread of the virus in at least two countries in one WHO region.

At Phase 6, the world has a new pandemic under way. It is characterized by community level outbreaks in at least one other country outside the WHO region identified in Phase 5.

However, experts note that moving into Phase 6 does not mean that the virus is any more deadly.

"By saying we are going to Phase 6, this would mean the spread continues," Keiji Fukuda, WHO's director of health security, said in a press conference on 9 June. "This does not mean the severity of the situation has increased."

It is hoped that the new classification of A(H1N1) will send a signal to governments and health officials to continue and improve contingency and containment plans for the possibility of large numbers of people catching the virus.