Читать книгу The Power of Plagues - Irwin W. Sherman - Страница 25

Figure 3.1 Plague in an Ancient City (detail) circa 1652-1654 by Michiel Sweerts (1624-1664).

As humans changed their lifestyles, their relationship with infectious diseases came to be altered. For 2 million years these human populations consisted of small groups of hunter-gatherers with limited contact with other such groups, and there were no domesticated animals. Such a population structure, with little or no exposure to new sources of infection and where parasite survival and transmission were minimized, led to a situation in which epidemic diseases were virtually nonexistent. Indeed, only those diseases with very high transmission rates that induced little or no immunity, as well as macroparasitic diseases that did not involve vectors for transmission and sexually transmitted diseases, were able to establish themselves in the groups of hunter-gatherers. Although some vector-borne diseases, such as malaria and yellow fever, may have been present at this stage of human history, it was only after human populations settled down and adopted an agricultural life, or continued a nomadic existence that depended on the husbandry of large herds of animals, that conditions favored the emergence of epidemic diseases (plagues). Historically, plagues (Fig. 3.1) came to be recorded only in our recent past, a time when we became farmers.

By 8000 B.C. the human population was settled in villages—first in the valleys of the Tigris and Euphrates Rivers in Mesopotamia and then along the Nile in Egypt, the Indus in India, and the Yellow River in China. Agriculture provided increased amounts of food for the people, but it also contributed to the conditions that would result in a decline in human health. It was the agricultural revolution, with the cultivation of crops and animal husbandry, that provided the driving force for the growth of cities (urbanization). Urban life also enhanced the transmission of certain diseases through the air and water; by direct contact; and by vectors such as snails, mosquitoes, and flies. The diseases of antiquity (5000 B.C. to A.D. 700) were characterized by parasites with long-lived transmission stages (e.g., eggs) as well as those involving person-to-person contact. Thus, most became established only when a persistent low level of infectious individuals could be maintained, i.e., were endemic; this required populations greater than a few hundred thousand.

Figure 3.2 The blood fluke Schistosoma, causative agent of the Pharoah’s Plague. A. Hieroglyphic; B. Calcified egg from a mummy; C. Schistosoma haematobium egg as seen with light microscope; D. Schistosoma mansoni egg with miracidium inside; E. adults in copula, as seen with a scanning electron microscope (from David Halton); F. ciliated miracidium as seen with the scanning electron microscope (courtesy of Vaughan South-gate); and G. cercaria, as seen with a scanning electron microscope (from David Halton). mw, male worm; fw, female worm; gc, gynecophoric canal