Between epidemics (which occur when large numbers of people in a community, region, or country come down with influenza), viruses circulate that are related to the viruses from the most recent epidemic. They infect people with varying levels of immunity developed from previous flu infections. After circulating and changing slowly over two to three years, the viruses have mutated enough to cause a new epidemic.⁸²

Type A viruses are more troublesome than B viruses, because Type A influenza virus can "shift" - that is, move from other species into humans. A novel strain emerges by reassortment (rearrangement) with circulating human strains, or by infecting people directly. Because they flourish in the face of global susceptibility, viruses that have undergone antigenic shift usually create pandemics.⁸³

Type B viruses are incapable of "shift". They can only "drift", which type A viruses also can do. "Drift" is a gradual change of the hemagglutinin or neuraminadase proteins on the surface of a particular strain of influenza. The change occurs in response to antibodies in people who have been exposed to it. "Drift" is an ongoing process in both types of virus, and it necessitates ongoing changes in influenza vaccines.⁸⁴