  antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. the term is often applied specifically to influenza, as that is the best-known example, but the process is also known to occur with other viruses, such as visna virus in sheep.) antigenic shift is a specific case of reassortment or viral shift that confers a phenotypic change. antigenic shift is contrasted with antigenic drift, which is the natural mutation over time of known strains of influenza (or other things, in a more general sense) which may lead to a loss of immunity, or in vaccine mismatch. antigenic drift occurs in all types of influenza including influenzavirus a, influenza b and influenza c. antigenic shift, however, occurs only in influenzavirus a because it infects more than just humans. affected species include other mammals and birds, giving influenza a the opportunity for a major reorganization of surface antigens. influenza b and c principally infect humans, minimizing the chance that a reassortment will change its phenotype drastically. antigenic shift is important for the emergence of new viral pathogens as it is a pathway that viruses may follow to enter a new niche. it could occur with primate viruses and may be a factor for the appearance of new viruses in the human species such as hiv. due to the structure of its genome, hiv does not undergo r
Influenza Virology