A viruses undergo two kinds of changes. One is a series of mutations that occur over time and cause a gradual evolution of the virus. In this case, a new subtype of the virus suddenly emerges. Influenza A and B viruses continually undergo antigenic drift. This process accounts for most of the changes that occur in the viruses from one influenza season to another.
Antigenic shift occurs only occasionally.
When it does occur, large numbers of people, and sometimes the entire population, have no antibody protection against the virus. This may result in a worldwide epidemic, called a pandemic. When this virus first emerged, it was associated with lower mortality than that caused by the two previous pandemic viruses. Several possible reasons for this lower mortality have been hypothesized. A second possibility is suggested by evidence that a virus with a similar hemagglutinin may have circulated from the late 1890s to the early 1900s.