Emerging infections: pandemic influenza.

The report of the Institute of Medicine's Committee on Emerging Microbial Threats to Health in the United States, published in 1992, defines influenza virus as the prototype emerging infection (1). Pandemics of influenza have been recognized since earliest recorded history and, because of the mutability of the virus, still represent a formidable threat to the health of the nation. Although much progress has been made in describing the molecular aspects of the virus, in elucidating the epidemiology and modes of spread, and in developing methods for prevention and treatment, a rational strategy for control has not been established. The trends of modern society, including the increasing availability of rapid human transportation and the urbanization of the rapidly expanding human population, tend to facilitate the spread of influenza and increase morbidity. Modern medicine can reduce the mortality that resulted from complications of infection with influenza virus during earlier epidemics, but the cost of medical interventions has increased to the point that effective methods of epidemic control should be considered. This challenge provides an opportunity to develop, test, and have in place a strategy for control of interpandemic influenza before the next pandemic. Pandemics result from the emergence of an influenza A virus that is novel for the human population. Evidence for recycling of subtypes of influenza A after intervals of 60 years or more has been derived by determining antibody prevalence in elderly populations prior to the emergence of subtypes H2N2 in 1957 and H3N2 in 1968 (2, 3). A more ominous threat is the reservoir of 14 influenza A subtypes that persist in avian hosts (4). An avian virus can reassort with a human virus, as occurred in 1957 and 1968, to allow the creation of progeny that possess novel surface antigens with the potential to spread in human popu-