The term pandemic refers to the spread of an infectious disease across countries and continents, i.e. the global spread of an epidemic. The latter is locally limited. The World Health Organization (WHO) distinguishes six phases of spread. They are defined by spatial and temporal factors:
Stage 1, low risk: No new virus subtypes detected in humans.
Stage 2, higher risk: A subtype circulating in the animal kingdom may pose a higher risk of disease for humans, but no new virus subtypes have yet appeared in humans.
Stage 3, alarm phase: The disease spreads to other countries and continents. People become infected only sporadically, especially in the country of origin and only with very close contact.
Stage 4: Smaller herds of human-to-human transmissions are detected in the country of origin and outside. The virus is still only partially adapted to humans.
Stage 5, significant pandemic risk: There are large numbers of human-to-human transmissions, including outside the country or continent of origin, but limited in time and location. At this stage, the virus is better adapted to humans, but not yet complete.
Stage 6, pandemic period: A virus is transmitted from human to human in the entire population worldwide.
The WHO warns that even a less devastating pandemic could place a heavy burden on health systems.