Illustration of antigenic shift (influenza) postcard

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. Image: National Institute of Allergy and Infectious Diseases (public domain)