Epidemiology and virology of influenza illness. Based on a presentation by Arnold S. Monto, MD.

Although researchers have demonstrated that influenza illness has affected societies for many centuries, in more recent decades, the World Health Organization and the Centers for Disease Control and Prevention have developed sophisticated means of tracking the virus' activity around the globe. The importance of tracking influenza activity lies not only in detecting what is circulating in other countries, but also in enabling global manufacturers of vaccine to prepare the appropriate A and B compositions for the impending epidemic. Tracking influenza also allows quantification of its great toll in terms of morbidity and mortality. Influenza has broad societal and financial impact, with as much as $12 to $14 billion in indirect economic impact in the United States annually and a significant increase in physician visits for febrile respiratory illness. For example, during many influenza outbreaks visits to family care physicians and internal medicine specialists can increase by 150% to 450%. Influenza presents an ongoing public health issue because of the virus' ability to continually reinvent itself. Antigenic drift, caused by subtle changes in influenza's surface proteins, partially accounts for annual epidemic outbreaks of the illness. Antigenic shift that occurs because of major changes in the viral hemagglutinin and sometimes in the neuraminidase, the other surface protein, results in the more widespread and lethal pandemic forms of influenza. Moreover, type A influenza is not an exclusively human infection. By understanding how both of these types of changes occur in waterfowl, the virus' primary nonhuman hosts, scientists can develop both drugs and epidemiological strategies to avert or minimize the most severe effects of influenza.