Pandemic

A pandemic (from Greek παν pan all + δήμος demos people) is an epidemic (an outbreak of an infectious disease) that spreads across a large region (like a continent), or even worldwide.

 Common killers and pandemics 
According to the World Health Organization, a pandemic exists when three conditions have been met:

 A disease emerges that is new to a population.
 The agent that causes the disease infects humans, causing serious illness.
 The agent spreads easily and sustainably among humans.

A disease or condition is not a pandemic merely because it is widespread or kills many people. It must also be infectious. For example, cancer causes many deaths, but it is not a pandemic because it is usually not infectious.

 World Health Organization pandemic phases 
The World Health Organization (WHO) has developed a plan to prepare the global fight against influenza. It defines the stages of a pandemic and makes recommendations about what countries should do before and during a pandemic. The phases are:

Interpandemic period:
 Phase 1: No new influenza virus subtypes have been detected in humans. 
 Phase 2: No new influenza virus subtypes have been detected in humans. However, a variant of the virus causes disease in animals. This means there is a threat of human disease.
Pandemic alert period:
 Phase 3: Humans get infected with a new subtype of the flu virus. However, the virus is not spreading from person to person.
 Phase 4: There are small cluster(s) of flu infection. Person-to-person spread is limited and localized.
 Phase 5: There are larger cluster(s) of flu infection. Person-to-person spread is still localized.
Pandemic period:
 Phase 6: Pandemic: The flu has spread throughout the general population. Person-to-person spread is increased and sustained.

Related pages
 Epidemic
 List of epidemics

 References 

 Steward's "The Next Global Threat: Pandemic Influenza".

 Other websites 
 WHO - Authoritative source of information about global health issues
 Past pandemics that ravaged Europe
 CDC: Influenza Pandemic Phases
 U.S. government's pandemic flu and avian flu information site