Pandemic preparedness – World Health Organization

2009.04.18 – “An influenza pandemic occurs when a new influenza virus appears against which the human population has no immunity, resulting in epidemics worldwide with enormous numbers of deaths and illness.”

https://web.archive.org/web/20090418142949/http://www.who.int/csr/disease/influenza/pandemic/en/

2009.05.20 – “A disease epidemic occurs when there are more cases of that disease than normal.”

https://web.archive.org/web/20090520211728/http://www.who.int/csr/disease/influenza/pandemic/en/