(E. Domingo et al., 1978). A virus, then, is identified as a consensus that reflects the predominating mutant(s) in a mixed population. Because predominating mutants seldom change, only unusual selective pressure by the host's immune response or other factors, such as host adaptation, will allow new mutants to gain ascendancy, resulting (rarely) in the emergence of distinguishably new viruses.

Although influenza viruses mutate at a rate similar to other RNA viruses, they are unique in that they also evolve (undergo meaningful changes) relatively rapidly in nature. This is due to selective pressure on the virus from the large population of partially immune people, who have antibodies to the virus as a result of previous infections. To survive, the virus must undergo some degree of genetic mutation (or "antigenic drift"). This process is continuous and results in regional epidemics of influenza.

Much less frequently, the surface proteins—called hemagglutinin (H) and neuraminidase (N)—of the influenza virus undergo a radical change (an "antigenic shift") that creates a virus so different that no person possesses immunity to it. A pandemic of life-threatening disease results. Interestingly, the radical changes that have produced pandemic influenza viruses are rooted in the virus's acquisition of genetic material from animal influenza A viruses. Scientists have hypothesized that agricultural practices in Southeast