FACTBOX: WHO's pandemic alert phases

(Reuters) - The World Health Organization (WHO) declared the first flu pandemic of the 21st century on Thursday.

WHO has used a series of six phases of pandemic alert as a system for informing the world of the seriousness of the threat and the need to launch progressively more intense preparations and measures.

The WHO revised their scale last April just days before the alert level was raised to the level of 5.

Here are some details on the highest levels:

* THE DECISION:

-- Margaret Chan, director-general of the United Nations agency in Geneva, is charged with deciding on when to move from one phase to another.

-- Her decision is based on advice from international public health experts.

-- The WHO emergency committee of experts has assessed the risk from a swine flu outbreak that has killed more than 140 people across the world.

* PHASES 5-6/PANDEMIC:

Widespread human infection:

* PHASE 5: The same virus causes sustained community-level outbreaks in two or more countries in one region.

"While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the Organization, communication, and implementation of the planned mitigation measures is short."

* PHASE 6:

The virus causes sustained community-level outbreaks in more than one region.

"Designation of this phase will indicate that a global pandemic is under way."

-- POST PEAK:

Possibility of recurrent events.

Pandemic levels dropping below peak levels, but countries need to be prepared for a second wave.

-- POST PANDEMIC:

Disease activity at seasonal levels.

Sources: Reuters/World Health Organization