Influenza is a highly contagious acute respiratory illness that appears to have caused serious disease in humans since ancient times. Many early accounts of epidemics of respiratory disease describe typical features of influenza including the clinical symptoms, short incubation period, high attack rates, and rapid progression of the disease through the population (1). Certain well-documented features of modern epidemics of influenza also emerge from these early accounts. Epidemics of varying severity occurred at regular intervals, caused the highest mortality in the elderly, and were thought to have first appeared in Asia. The quest for the causative agent of influenza was intensified after the devastating pandemic of" Span ish influenza" in 1918 and 1919, to which was attributed 20 to 40 million deaths (1). The first isolation from humans of influenza A virus occurred in 1933 and of influenza B in 1940. Although more than 50 years have passed, influenza viruses continue to cause considerable excess mortality