The early history of epidemiology is closely linked to the history of infectious diseases and can be divided into three distinct periods. The earliest period, which can be traced to the writings of Hippocrates in the third and fourth centuries B.C., was that of clinical description of diseases with little investigation into their specific characteristics and etiologies. The second period, spanning the eighteenth and nineteenth centuries, may be distinguished by the rejection of early Hippocratic and Galenic doctrines and the more systematic description of morbid conditions. The third period, which was marked by the discovery of specific microbial causes of disease, spurred an extraordinary growth of knowledge and scientific exploration. This period will be the main focus of the paper, as it had the greatest influence on the development of American infectious disease epidemiology.