Free Plagues and Peoples by William H. McNeill

the result is hyperinfestation of the denuded landscape by a limited number of different kinds of weeds. Yet weeds do not prevail for long in nature. Complex compensatory adjustments soon manifest themselves, and in the absence of fresh “external” disturbances of a far-reaching sort, a more or less stable and variegated flora will re-establish itself, usually looking much like what had been destroyed at the start.
    But as long as human beings continued to expend effort to alter natural landscapes and fit them for agriculture, they prevented re-establishment of natural climax ecosystems, and thereby kept open the door for hyperinfestation. 4 As we have seen, when dealing with relatively large-bodied organisms that humans could see and manipulate, observation and experiment soon allowed early farmers to keep weeds (as well as animal pests like mice) under control. But human intelligence remained for thousands of years only fumblingly effective in dealing with disease-causing micro-organisms. As a result, the ravages of disease among crops, herds, and peoples played a significant part in human affairs throughout historic time. In fact, the effort to understand what happened in a way that humans could not do before modern medical discoveries made clear some of the important patterns of disease propagation is the
raison d’être
of this book.
    So far so good. But when one seeks to descend from this level of generalization and ask what sorts of disease arose or extended their sway in what parts of the world and at what times and with what consequences for human life and culture, uncertainty blocks any adequate answer. Even if one excludes diseases affecting crops and domestic animals, exact information is lacking wherewith to create a history of human infections.
    It is easy to see that settling down to prolonged or permanent occupancy of a single village site involved new risks of parasitic invasion. Increased contact with human feces as they accumulated in proximity to living quarters, for instance, could allow a wide variety of intestinal parasites to move safelyfrom host to host. By contrast, a hunting band, perpetually on the move with only a brief sojourn in any location, would risk little from this kind of infectious cycle. We should expect that human populations living in sedentary communities were therefore far more thickly infested with worms and similar parasites than their hunting predecessors or contemporaries in the same climatic zones. Other parasitic organisms must have found it easy to move from host to host via contaminated water supplies. This, too, was far more likely to happen when human communities remained in one location permanently and had to rely on the same water sources for all household needs year in and year out.
    All the same, the small village communities characteristic of earliest agriculture may not always have fallen prey to particularly heavy parasitic invasion. Near Eastern slash-and-burn cultivators moved from place to place several times in a lifetime; Chinese millet farmers and Amerindian cultivators of maize, beans, and potatoes were scattered rather thinly and lived in small hamlets during pre-civilized times. Infections and infestations of various sorts presumably established themselves in these communities, and, although the parasite population must have differed from place to place, within each village or hamlet nearly everyone probably acquired about the same assortment of parasites in youth. Such, at any rate, is the case today among primitive cultivators. 5 Yet such infections cannot have been a very heavy biological burden, since they failed to inhibit human population growth of unexampled magnitude. Within only a few hundred years, in all the historically significant regions where valuable food crops were successfully domesticated, human population density became ten to twenty times greater than hunting densities had ever been in the same areas. 6
    Insofar as early agriculture