- Defined by the World Health Organization to include 6 stages of development. At stage 6, a pandemic is is reached.
A pandemic is the rapid spread of disease to a large population spread across a large region outside of normal local boundaries within a short period of time (1-2 weeks)
Phase I is the period before any new infectious subtypes have been detected in animals, a stage that is often missed since new subtypes are occur yearly
Phase II begins after the new virus has been shown to infect humans
Phase 3 begins when cases of the new virus appear in humans throughout the world for example during H5NI virus, or avian flu, a disease that spread from birds to humans but not from humans to humans. 
Phase IV occurs when the virus can easily transmit between humans and results in this results in community-level epidemics
Phase V involves disease spread across continents and WHO regions and often is inevitibel until true pandemic is reached.
Phase VI is the definition of a pandemic and requires documentation of the disease in multiple continents or two WHO regions
- WHO H5N1http://www.who.int/influenza/human_animal_interface/en/
- WHO Disease Regions http://www.who.int/healthinfo/global_burden_disease/definition_regions/en/