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. Federal experts in public health and vaccine development are working to reduce the impact of flu each year, as well as helping to coordinate national preparations for the eventuality of a flu pandemic or outbreak of a new influenza strain. Researchers at CDC, the National Institutes of Health, and the Food and Drug Administration work together to identify new strains of influenza and develop annual vaccines. In addition to the United States, flu pandemic planning includes representatives from the World Health Organization, and ministries or departments of health from a number of other countries.