Abstract
A disease epidemic occurs when there are more cases of that disease than normal. A pandemic is a worldwide epidemic of a disease.
An influenza pandemic may occur when a new influenza virus appears against which the human population has no immunity. The World Health Organisation (WHO) has defined the phases of a pandemic to provide a global framework to aid countries in pandemic preparedness and response planning.
Pandemics can be either mild or severe in the illness and death they cause, and the severity of a pandemic can change over the course of that pandemic.
World Health Organisation