How do influenza pandemics emerge?
A pandemic can emerge when the influenza A virus changes suddenly – what experts call an antigenic shift. The HA and/or NA proteins, which are on the surface of the virus, have new combinations; resulting in a new influenza A virus subtype. This new influenza subtype needs one characteristic to cause a pandemic – it must be easily human transmissible (it can easily spread from one person to another). After the pandemic has emerged and spread, the virus subtype circulates among humans for several years, causing occasional flu epidemics. These will not usually become more than epidemics because humans have developed some immunity over time. Various bodies around the world, such as the Health Protection Agency (UK), the World Health Organization (WHO), and the Centers for Disease Control and Prevention (USA) monitor the behavior and movements of the virus.
