Template:POTD/2011-05-19
An illustration explaining how antigenic shift can occur in the influenza virus. Antigenic shift occurs when two or more different strains of one or more viruses combine to form a new subtype having a mixture of each original virus's surface antigens. The process may occur in any number of viruses, but influenza is the best-known example. Antigenic shift is a specific case of reassortment or viral shift that confers a phenotypic change, and should not be confused with antigenic drift, which is the natural mutation over time of known viral strains.