The name "influenza" usually conjures up a picture of the disease as it appeared during the pandemic of 1918-1919—a disease of widespread distribution and protean manifestations, characterized by rapid dissemination, a high frequency of complicating pneumonia, and a high mortality. In sharp contrast, one may present the milder epidemics that occur from year to year in different communities, usually limited in their scope, severity and complications. The number of cases that occur in these mild epidemics depends, to a certain extent, on the number of clinically similar infections that are included under the diagnosis of influenza.
THE DIAGNOSIS OF INFLUENZA
In fact, the lack of sharp differential features has rendered it difficult to separate influenza as a well defined clinical entity from many other mild infections of similar symptomatology that may invade a community. Thus the term "influenza" has been carelessly applied to various vague diseases of comparatively high morbidity
FRANCIS T. RECENT ADVANCES IN THE STUDY OF INFLUENZA. JAMA. 1935;105(4):251-254. doi:10.1001/jama.1935.02760300011003