Plagues and Peoples by William H. McNeill

cyclic equilibrium. Seasonal changes in temperatures and moisture, for instance, tend to concentrate childhood diseases in modern cities of the temperate zone in spring months.
    The number of susceptible persons in a population is also fundamental, as are the ways in which they congregate or remain apart. School and military service, for example, havebeen the two most significant ways susceptible youth congregate in modern times. Any parent can attest the role of primary schools in propagating childhood diseases in contemporary western societies: in the nineteenth century, before inoculations became standard, draftees into the French army from the countryside suffered—sometimes seriously—from infectious disease to which their city-bred contemporaries were almost immune, having already been exposed. As a result, robust peasant sons had a far higher death rate in the army than did undernourished weaklings drafted from urban slums. 35
    The size of dose required to infect a new host, the length of time during which the infection may be transferred from one person to another, modes of such transfer, and customs affecting opportunities for exchanging infections, all play roles in determining how many get sick and when. Not infrequently a disease requires a massive, megalopolitan concentration of human hosts to survive indefinitely. In such a population the chance of encountering enough susceptible new hosts so as to keep a chain of infection going is obviously greater than when potential hosts are scattered thinly across a rural landscape. Yet when enough susceptible persons exist in rural communities, such a disease can sally forth from its urban focus and run like a terrifying brushfire from village to village, household to household. Such outbreaks, however, fade away as rapidly as they arise. As the local supply of susceptible hosts runs out, the infection dies and disappears, except in the urban center whence it had initially emerged. There, enough susceptibles will remain for the infectious organism to keep itself alive until disease-inexperienced individuals again accumulate in the rural landscape and another epidemic flare-up becomes a possibility.
    All these complex factors sometimes shake down to relatively simple over-all patterns of incidence. Careful statistical study of the way measles propagates itself in modern urban communities shows a wave pattern cresting in periods of time just under two years. Moreover, it has recently been demonstratedthat to keep this pattern going, measles requires a population with at least 7,000 susceptible individuals perpetually in its ranks. Given modern birth rates, urban patterns of life and the custom of sending children to school, where measles can spread very rapidly through a class of youngsters meeting the virus for the first time, it turns out that the minimal population needed to keep measles going in a modern city is about half a million. By scattering out across a rural landscape, a smaller population suffices to sustain the chain of measles infection. The critical threshold below which the virus cannot survive falls between 300,000 and 400,000 persons. This can be demonstrated by the way the measles infection behaves among island populations ranking above and below this critical mass. 36
    No other disease current in our own time exhibits so precise a pattern, and none, probably, requires such large human communities for its survival. Comparably exact studies for other common childhood diseases have not been made, largely because artificial immunization procedures have altered patterns of infection in far-reaching ways in all modern countries. Yet notable changes in virulence as well as in the frequency of the most common childhood diseases have occurred as recently as the nineteenth century, when European governments first started to collect statistics on the incidence of separate infectious diseases. In other words, the adjustment between the disease-causing organisms