A pandemic occurs when an epidemic crosses international borders affecting a large number of people. The key to an influenza pandemic developing is when a new strain of the virus begins to circulate and most of the population has no immunity. This may result in severe disease. The World Health Organization has a more specific definition for when they declare an influenza pandemic. This definition focuses on the virus spreading from person to person (as opposed to from animals to people) and the epidemic existing across different regions of the world.