Influenza

Definition: Influenza is a virus commonly called the flu and has two types of viruses, A and B. Influenza A is further subtyped according to the type of hemagglutinin (H or HA) and neuraminidase (N or NA) surface proteins it shows on the outside of the virus.

These surface proteins are antigens, they provoke an immune response in the body. Although immunity to either the hemagglutinin or the neuraminidase surface proteins of an influenza A virus protects a person somewhat from getting sick, the hemagglutinin protein provokes a stronger immune response.

The Influenza B virus also has surface proteins that provoke an immune response in the body. Unlike Influenza A, Influenza B surface antigens to not change rapidly.

Due to the constant changing of the antigen in Influenza A viruses, humans become infected over and over again with "new" strains of the flu. All pandemics have been caused by Influenza A viruses. The Influenza A subtype H5N1 bird flu is a possible candidate for the next pandemic flu.

According to the Center for Disease Control and Prevention (CDC) in Atlanta, Influenza A/ H1N1, H3N2 and H1N2 and Influenza B viruses are the most frequent flu subtypes and types we see today. A different mixture each year of three of these viruses (two Influenza A and one Influenza B) make up the seasonal flu shot.

Examples: Sources:

• Advisory Committee on Immunization Practices. Prevention and Control of Influenza. MMWR, July 29, 2005. 54(RR08):1-40.
• Center for Infectious Disease Research and Policy, Infectious Diseases Society of America. Avian Influenza (Bird Flu): Implications for Human Disease. Updated June 2, 2006.