B. Type A viruses can be subtyped based on the surface protein hemagglutinin and neuraminidase. Influenza viruses can further be subdivided into strains. A virus that people have not been exposed to and, therefore, do not have protective immunity against.
Influenza viruses undergo two kinds of change.
One is a series of mutations over time that cause a gradual evolution of the virus, known as drift which results in the emergence of new strains . The other is an abrupt change in the surface proteins, known as antigenic shift which results in new subtypes of influenza A. Flu pandemics have been recognized since the 1500s. Birds are the primary reservoir for new influenza viruses. A subtypes have been found in birds. The impact of the next pandemic could have a devastating effect on the health and well being of the American public.