Plagues and Peoples by William H. McNeill Page B

peculiar hallmark and epidemiological burden of cities and of countryside in contact with cities. They are familiar to almost all contemporary humankind as the ordinary diseases of childhood: measles, mumps, whooping cough, smallpox and the rest. 22
    Contemporary global diffusion of childhood diseases required several thousand years to establish itself, and a good part of the subject matter of this book will be a consideration of critical thresholds in that diffusion process. Moreover, one must suppose that the initial establishment of these diseases (or of infections ancestral to those we know today) was itself a gradual process, involving numerous false starts and lethal encounters in which either the human hosts or the invading parasite died out locally, and thus broke off the chain of infection before it could become a normal, endemic, more or less stable element in the biological balances of civilized human life.
    Most and probably all of the distinctive infectious diseases of civilization transferred to human populations from animal herds. Contacts were closest with the domesticated species, so it is not surprising to find that many of our common infectious diseases have recognizable affinities with one or another disease afflicting domesticated animals. Measles, for example, is probably related to rinderpest and/or canine distemper; smallpox is certainly connected closely with cowpox and with a cluster of other animal infections; influenza is shared by humans and hogs. 23 Indeed, according to a standard handbook,diseases human populations share today with domestic animals number as follows 24 :
     
Poultry
26
Rats and mice
32
Horse
35
Pig
42
Sheep and goats
46
Cattle
50
Dog
65
     
    There are many overlaps in this tabulation, since a single infection often afflicts several animal species as well as humans. Moreover, because some infections are rare while others occur commonly, a mere listing of the variety is not very significant. Nevertheless, the number of overlaps does suggest how ramified our disease relations with domesticated animals have become. It also appears obvious that the sharing of infection increases with the degree of intimacy that prevails between man and beast.
    In addition to diseases derived from or shared with domesticated animals, human populations may become diseased by intruding upon one or another disease cycle established among wild animals. Bubonic plague, at home among burrowing rodents, yellow fever at home among monkeys, and rabies at home among bats are examples of the more lethal of such infections. 25
    Novel transfers of parasites from one host to another have not ceased to occur, and even in recent times such events have sometimes had abrupt and drastic consequences. Rinderpest invaded Africa in 1891, for instance, where it killed off very large numbers of domesticated cattle as well as antelope and other wild species; but its ravages were so severe and sudden—up to 90 per cent die-off occurred—that the disease did not establish itself as an endemic. 26 Instead, it disappeared after a few years, presumably from lack of susceptible surviving populations of ungulates to infect. In 1959 a new human disease, called O’nyong nyong fever, appeared in Uganda,probably as a result of the transfer of a virus from monkeys. The disease spread rapidly and widely, but in this case its effects upon human beings were mild, and recovery (with the development of suitable immunity) came quickly. As a result, O’nyong nyong fever, like rinderpest among African antelopes, failed to establish itself as an endemic human infection. Instead, it disappeared as mysteriously as it had come, presumably by retreating back into the treetops, where it was properly at home. 27 A decade later, in 1969, another fever, far more lethal than the Ugandan outbreak, manifested itself in Nigeria. Termed Lassa fever from the hospital station where it was first noticed by western-trained doctors, the new disease was