Influenza pandemics and avian flu

摘要

The word pandemic is used to describe a disease that is epidemic throughout the world at more or less the same time. The other criterion for defining a pandemic relates to the causative virus. A pandemic occurs when a completely new virus emerges-a virus that shows a more radical change (antigenic shift) than the change occurring continuously in influenza viruses (antigenic drift) and which is generally associated with more severe illness. Britain is used to experiencing flu in most winters, but these outbreaks are not pandemics because they are not consistently present in all countries at the same time and not caused by new virus.