Emerging influenza viruses: past and present.

Influenza is an example of a disease for which the viral pathogen has emerged into the human population many times over past centuries, sometimes with devastating consequences [1]. Historical records provide vivid descriptions of past influenza outbreaks, and the viruses that caused the pandemics of the last century remain subjects of great interest. It is almost certain that a new pandemic, caused by the zoonotic transmission of a new influenza virus into humans, will occur. The recent outbreaks of the highly pathogenic H5 and H7 subtype viruses in poultry and their limited transmission into humans, as well as transmission of H9 subtype viruses, have raised concerns that conditions are developing for the generation of a new pandemic virus. In this paper, we review past pandemics, viral determinants of cross-species transmission of viruses, molecular factors that contribute to disease, and preventative measures to reduce the impact of a future pandemic.