While the determinants of epidemics are undoubtedly complex, they are understandable. In this lecture on interactions between viral pathogens and their hosts, Dr. Ganem will consider some of the forces that create and shape epidemic infection. New infectious diseases usually result from an upset in a previously established balance between the virus and its natural host. Mutations or other genetic changes in the virus, for example, may confer upon it new properties, such as the ability to grow more efficiently in its natural host or to spread to a species that is not ordinarily susceptible. The most dramatic recent example of the hazards of genetic change in a virus is the Hong Kong influenza outbreak of 1997-1998, in which a novel avian flu virus emerged in poultry flocks and spread to humans.
Viral mutation is only one way to upset the balance.
Changes in the environment, either natural or manmade, can also have disruptive effects. Such disturbances can, for example, bring a virus into contact with human hosts who had rarely encountered it in the past. Many new epidemics require both genetic change in the pathogen and a shift in exposure patterns between humans and viruses. Once established, emerging viruses and their hosts continue to coevolve over time, ultimately establishing a new balance. In this equilibrium, the virus' drive to grow rapidly is tempered by the fact that anything that shortens the life of the host will curtail opportunities for the virus to spread.