Plagues and Peoples by William H. McNeill Page A

five to ten thousand years ago, it is no longer possible to infer or observe, as one still can do in Africa, the disease costs which particular agricultural inventions and territorial expansion may have involved.
    We can, nevertheless, infer one important general alteration in disease exposure that came, sooner or later, to all civilized communities. Eventually agricultural populations became dense enough to sustain bacterial and viral infections indefinitely, even without benefit of an intermediate nonhuman host. This cannot ordinarily happen in small communities, since unlike multicelled parasites, bacterial and viral invasions provoke immunity reactions within the human body. Immunity reactions impose drastic alternatives upon the host-parasite relationship. Whenever they dominate the interaction of host and parasite, either speedy death of the infected person or full recovery and banishment of the invading organism from the host’s body tissues ensues—at least for a period of time of months or years until the immunizing antibodies fade from the bloodstream so as to permit reinfection.
    As usual in biology, things are not quite so simple as such a statement implies. Individual resistance to infection is notsimply and solely a matter of the formation of antibodies. In some cases, moreover, even infections that do provoke antibodies may linger on for years or even throughout a lifetime. Individual “carriers,” like the famous “Typhoid Mary,” may harbor a disease organism indefinitely and experience no very noticeable ill effects themselves while communicating the infection to others with drastic, even fatal, results. In still other cases, an infection may become “latent,” that is, withdraw to some region of the host’s body and hide there for lengthy periods of time.
    One of the most remarkable patterns of latency allows the chicken pox virus to disappear for as much as fifty years, by retreating into the tissues of the efferent nerves, only to reappear as an affliction of the elderly known as shingles. In this way, the virus neatly solves the problem of maintaining an unbroken chain of infection within a small human community. Even if every available human host gets the chicken pox and develops immunity so that the disease disappears, still, decades later, when a new generation of susceptible human beings has had time to come into existence, the infection can recur, creeping down the efferent nerve paths to the skin of an elderly member of the community, and there manifesting itself as shingles. Transferred to a new host, however, the virus provokes the familiar childhood symptoms of chicken pox. Both the mildness of the disease for most people and the remarkable latency pattern it exhibits suggest that this is an old viral infection among humankind. In this respect chicken pox is unlike the other common childhood diseases of modern times. 21
    Diseases that lack such a technique of survival and yet confront the drastic alternatives created by antibody reactions within the host’s body have to rely on numbers for their survival. Numbers, that is, of potential hosts, among whom, if the total size of the community is sufficient, there will always be someone who has not yet had the disease and therefore remains susceptible to infection. Such parasites, are, in all probability, rank newcomers in the time scale of biologicalevolution, even if ancient and immemorial on the time scale of human history. Only in communities of several thousand persons, where encounters with others attain sufficient frequency to allow infection to spread unceasingly from one individual to another, can such diseases persist. These communities are what we call civilized: large, complexly organized, densely populated, and without exception directed and dominated by cities. Infectious bacterial and viral diseases that pass directly from human to human with no intermediate host are therefore the diseases of civilization par excellence: the