Introduction

Influenza is the paradigm of a viral disease that relies on continued evolution of the virus to cause annual epidemics and occasional pandemics of disease in humans. The gene pool of influenza A viruses in aquatic birds provides all the genetic diversity required for the emergence of pandemic influenza virus for humans, lower animals and birds. In humans, pigs and horses, influenza A viruses show both antigenic drift and genetic shift (Webster et al., 1992). In contrast, there is emerging evidence that avian influenza viruses are in evolutionary stasis. This review reports that rapid evolution in influenza A viruses in humans and other mammals has continued since the beginning of recorded medical history and depends on periodic introductions of gene segments or entire influenza viruses from the avian influenza virus gene pool. In aquatic wild birds, influenza virus appears to be fully adapted to its host and causes no disease signs. Thus, understanding the ecology of influenza viruses in the reservoir of aquatic birds is essential if we wish to find ways to intervene and reduce or prevent the occasional catastrophic pandemics such as the one that decimated the human population of the world in 1918 after the appearance of ‘Spanish’ influenza.

Host-specific evolution of influenza viruses

It is likely that the NP protein gene determines host range (Scholtissek et al., 1985; Tian et al., 1985; Snyder et al., 1987); consequently NP gene evolution and host taxonomy has been studied (Gorman et al., 1990a, 1991; Gammelin et al., 1990).