Pandemic refers to an epidemic that has spread to several countries or continents, generally affecting a large number of people (CDC- Centers for Disease Control and Prevention).
or
It is an epidemic that reaches a wide geographical area and affects an exceptionally high proportion of the population (Merriam Webster Dictionary).
The H1N1 virus, for example, was responsible for the first pandemic of the century, declared by the WHO in 2009 (popularly referred as the swine flu). For some time, this disease was presented as having a rather high lethality, but after a few years the estimate was revised to around 0.02%. Although it has proven to be a less lethal disease, a pandemic did occur.
There is no relationship between the term and the dangerousness of the disease that causes it.
References
Definition of Pandemic by Merriam Webster Dictionary
Maria D. Van Kerkhove, Siddhivinayak Hirve, Artemis Koukounari, Anthony W. Mounts. Estimating age‐specific cumulative incidence for the 2009 influenza pandemic: a meta‐analysis of A(H1N1)pdm09 serological studies from 19 countries. Wiley Online Library, 2013.
Principles of Epidemiology in Public Health Practice, Third Edition
