INFECTIOUS DISEASES RESULT from the interplay of three main factors: the host, the parasite, and the environment. The matter is highly complex, since each of these factors can vary in many ways and many differing diseases can result. In this review, attention will be concentrated on two of these factors: the prehuman and human hosts and their environments during the periods of man's evolution from the earliest days to the present. A number of distinct eras can be discerned: those of the primate precursors of man, early man, agricultural man, industrial man, and the man of one world. The first three of these eras will be discussed here.