In a typical year, influenza claims between 30,000 and 50,000 victims in the United States, most of them the very young or the very old. About 10 times in the past 300 years, an epidemic has struck. The worst of these was probably the Spanish Influenza pandemic (or worldwide epidemic) of 1918. I remember my father vividly describing the deaths of people he knew and his own feeling, when he fell ill, that he too would die. The influenza strain that attacked in 1918 (forensic virologists are just beginning to identify it) had three special characteristics: 1) it was highly infectious and easily transmitted; 2) it caused serious illness with a high mortality rate; and 3) the mortality peaked among adolescents and young adults, unlike any other epidemic of this disease.

Influenza viruses are divided into three major types (A, B, and C) and, like all the other important epidemics of influenza, the Spanish Flu strain is classified as belonging to type A. This type is further subdivided into one of about 75 possible subtypes, and these are further divisible into strains. Being infected by one strain confers protection against another infection by a similar strain if the two are closely related. The success of influenza virus as a pathogen is the direct result of its genetic instability. As soon as a population contains many people who have been infected by one strain and have developed immunity to it, a new kind of flu for which there is little protection is likely to appear and sweep through the population.