Emergence of influenza A
Understanding emerging infectious diseases requires considering changes in the population ecology of hosts and evolutionary changes in the parasite. In the past, disease emergence has usually been viewed through the lens of either epidemiology, which ignores the possibility of evolution, or evolutionary optimization, which ignores population dynamics and transient evolutionary states. These perspectives may be too restricted especially for highly mutable parasites in rapidly changing environments. In such cases, ecological and evolutionary dynamics may interact over short time scales. Influenza A viruses provide relevant examples of how these processes may jointly determine host range. I will summarize recent worldwide changes in the ecology and evolution of these viruses in their major host populations, including waterfowl, poultry, swine, and humans. I will then introduce a model that explores how one evolutionary constraint of host range, the virus’s preference for a sialic acid receptor, interacts with changing ecological conditions to affect the probability of emergence, re-emergence, and adaptation of the virus in different host species.