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1 " We remain part of the earth's ecosystem, and participate in the food chain whereby we kill and eat various plants and animals, while our bodies provide a fair field full of food for a great variety of parasites. "
― William H. McNeill , Plagues and Peoples
2 " It is obvious that human (and non-human) diseases are evolving with an unusual rapidity simply because changes in our behaviour facilitate cross-fertilization of different strains of germs as never before, while an unending flow of new medicines (and pesticides) also present infectious organisms with rigorous, changing challenges to their survival. "
3 " ...if human numbers increase, the rate of infection also increases. "
4 " ...whenever a new, especially successful form of an infection emerges, it will spread rapidly around the globe. "
5 " Nonetheless, the way infectious diseases have begun to come back shows that we remain caught in the web of life - permanently and irretrievably- no matter how clever we are at altering what we do not like, or how successful we become at displacing other species. "
6 " 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. "
7 " For Europe, as for other civilized lands, infections by familiar epidemic disease surely became more frequent, at least in the major ports and at other foci of communication; but infections that returned at more and more frequent intervals became, by necessity, childhood diseases. Older persons would have acquired suitably high and repeatedly reinforced levels of immunity through prior exposures. Thus by a paradox that is only apparent, the more diseased a community, the less destructive its epidemics become. Even very high rates of infant mortality were relatively easily borne. The costs of giving birth and rearing another child to replace one that had died were slight compared to the losses involved in massive adult mortality of the sort that epidemics attacking a population at infrequent intervals inevitably produce.Consequently, the tighter the communications net binding each part of Europe to the rest of the world, the smaller became the likelihood of really devastating disease encounter. Only genetic mutation of a disease-causing organism, or a new transfer of parasites from some other host to human beings offered the possibility of devastating epidemic when world transport and communications had attained a sufficient intimacy to assure frequent circulation of all established human diseases among the civilized populations of the world. Between 1500 and about 1700 this is what seems in fact to have occurred. Devastating epidemics of the sort that had raged so dramatically in Europe's cities between 1346 and the mid-seventeenth century tapered off toward the status of childhood diseases, or else, as in the case of both plague and malaria, notably reduced the geographic range of their incidence.The result of such systematic lightening of the microparasitic drain upon European populations (especially in northwestern Europe where both plague and malaria had about disappeared by the close of the seventeenth century) was, of course, to unleash the possibility of systematic growth. This was, however, only a possibility, since any substantial local growth quickly brought on new problems: in particular, problems of food supply, water supply, and intensification of other infections in cities that had outgrown older systems of waste disposal. After 1600 these factors began to affect European populations significantly, and their effective solution did not come before the eighteenth century - or later.All the same, the changing pattern of epidemic infection was and remains a fundamental landmark in human ecology that deserves more attention than it has ordinarily received. On the time scale of world history, indeed we should view the 'domestication' of epidemic disease that occurred between 1300 and 1700 as a fundamental breakthrough, directly resulting from the two great transportation revolutions of that age - one by land, initiated by the Mongols, and one by sea, initiated by Europeans. "
8 " Something like a stable balance between human numbers and resources may then begin to define itself. "
9 " In any effort to understand what lies ahead, as much as what lies behind, the role of infectious disease cannot properly be left out of consideration. Ingenuity, knowledge, and organization alter but cannot cancel humanity's vulnerability to invasion by parasitic forms of life. Infectious disease which antedated the emergence of humankind will last as long as humanity itself, and will surely remain, as it has been hitherto, one of the fundamental parameters and determinants of human history. "
10 " ...the galloping increase of human numbers practically guarantees that existing margins between food supplies and human hunger will swiftly disappear, leaving less and less in reserve for times of unusual crisis. "
11 " ...it now requires an act of imagination to understand what infectious disease formerly meant to humankind, or even to our own grandfathers. "
12 " More generally, religious pilgrimages rivaled warfare in provoking epidemic infection. The doctrine that disease came from God could easily be interpreted to mean that it was impious to interfere with God's purposes by trying to take conscious precaution against disease, either in war or on pilgrimage. Part of the meaning of pilgrimage was the taking of risks in pursuit of holiness. To die en route was, for the pious, and act of God whereby He deliberately translated the pilgrim from the hardships of life on earth into His presence. Disease and pilgrimage were thus psychologically as well as epidemiologically complementary. The same may be said of war, where risk of sudden death -one's own or the enemy's- was at the very core of the enterprise. "