Viral mutations help the virus to survive through small or large changes in its antigenic sites. Most viruses mutate gradually over time to improve survival. Influenza viruses can mutate through antigenic shifts and antigenic drifts. Antigenic drift is common in both type A and type B. The HA (hemagglutinin) and NA (neuraminidase) proteins are the main sites for antibody recognition. The RNA segments that encode these proteins mutate slightly so that they are less recognizable by the antibodies. Antigenic shifts are when whole segments of RNA have been reassorted. Shifts only occur in Influenza type A. This phenomenon occurs when there are two types of Influenza A circulating in the body at the same time. The shifts usually occur in areas where humans and fowl or swine live close to each other. When these shifts occur, most people do not have immunity and a pandemic occurs. There have only been four Influenza pandemics in the last 100 years, with the most recent occurring in 2009.
