Changes Through Shift and Drift

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