GENEVA -- The World Health Organization on Wednesday raised the pandemic alert level from phase four to phase five, signaling that a pandemic is "imminent".

Here are the six different "pandemic influenza phases" defined by the WHO:

Phase 1: A virus circulates among animals but with no cases reported of infections in humans.

Phase 2: An animal flu virus is known to have caused infection in humans, and therefore considered a potential pandemic threat.

Phase 3: An animal or human-animal flu virus has caused sporadic cases or small outbreaks in humans, but has not resulted in human-to-human transmission that is sufficient to sustain community-level outbreaks.

Phase 4: Human-to-human transmission of an animal or human-animal flu virus that is able to cause community-level outbreaks. Significant increase in risk of a pandemic. The focus of this phase is to contain the spread of the virus.

Countries would be asked to take action such as issuing travel advice.

Countries affected by the disease should also consider deploying a pandemic vaccine, and limiting non-essential movement of people from containment areas.

Phase 5: Human-to-human spread of the virus into at least two countries in one region. Strong signal that pandemic is imminent.

Countries are required to advise people with respiratory illnesses to stay home. Classes should be suspended and work patterns modified.

Phase 6: Pandemic phase, with human-to-human spread of virus in at least two countries in the same geographic region and extending to at least one other country outside the region.

Implementation of contingency plans for health systems at all levels.