Plagues and Peoples Read Online Free Page B

parasites, digesting in their turn whatever they find nourishing within the human body.
    This is, however, only one facet of the exceedingly complex processes that affect the success or failure of any particular organism in invading and proliferating within a particular human being. In fact, despite all the advances of medical research in the past hundred years or so, no one fully understands their interaction. At every level of organization—molecular, cellular, organismic, and social—one confronts equilibrium patterns. 2 Within such equilibria, any alterationfrom “outside” tends to provoke compensatory changes throughout the systems so as to minimize over-all upheaval, though there are always critical limits which, if transgressed, result in the breakdown of the previously existing system. Such a catastrophic event may involve dissolution into simpler, smaller parts, each with equilibrium patterns of its own; or, on the contrary, may involve incorporation of smaller parts into some larger or more complex whole. The two processes may in fact combine, as in the familiar case of animal digestion, whereby the feeder breaks down the cells and proteins of its food into simpler parts only to combine them into new proteins and the cells of its own body.
    Simple cause-and-effect analysis is inadequate for such systems. Since many variables are simultaneously at work, interacting constantly and altering their magnitudes at irregular rates, it is usually misleading to concentrate attention on a single “cause” and try to attribute a particular “effect” to it. Study of simultaneity among multiple processes is presumably a better way to approach an understanding. But the conceptual and practical difficulties here are enormous. Recognition of patterns, and observation of their endurance or dissolution is, at most levels of organization, about as much as people are capable of; and at some levels, including the social, there is profound uncertainty and dispute about which patterns are worth attending to, or can, in fact, be reliably detected. Divergent terminologies direct attention to different patternings; and finding a logically convincing test, acceptable all around, that can determine whether one such system of terms is superior to its rivals, is often impossible.
    Yet the slow processes of evolution presumably apply to human societies and their symbolic systems as much as to human bodies, so that when logic cannot decide, survival eventually will. Terms that direct attention to the critically useful facets of a situation clearly do have enormous survival value for human beings. It is this aspect of our capacity to communicate with one another that has allowed
Homo sapiens
to become such a dominant species. Yet no system of terms isever likely to exhaust or completely comprehend all aspects of the reality around us. We have to do the best we can with the language and concepts we inherit, and not worry about obtaining a truth that will satisfy everyone, everywhere, and for all time to come.
    Just as language is a social and historical product, so, too, within wide limits is the very concept of disease. Holy men whom Americans today would consign to hospitals for the mentally ill abound in the historic record. Conversely, nearsightedness and a dull sense of smell, which we regard as compatible with good health, would probably have been classed as crippling diseases by our hunting ancestors. But despite such variability, there remains a firm and universal nucleus to the concept of disease. A person who can no longer perform expected tasks because of bodily disorder will always seem diseased to his fellows; and many such bodily disorders arise from encounters with parasitic organisms.
    To be sure, different human beings and entire communities exhibit widely varying levels of susceptibility and/or immunity to infections. Such differences are sometimes hereditary, but more often they are the result of past exposures to invading