person to person increased considerably with the passage of time as the sedentary communities in these four areas grew into civilizations. Hence, by about 500 B.C., each of the major civilized regions of Eurasia had developed its own unique variety of infections, person-to-person diseases. Within each region, the people built up an immunity to the diseases indigenous to it. McNeill thus attributes the territorial expansion of civilized peoples, in part, to the spread of their indigenous diseases to neighbors in adjacent sparsely populated areas who were unaccustomed to and unable to withstand such a vast array of infections.

McNeill contends—on the basis of very fragmentary and questionable evidence—that shortly after the beginning of the Christian era, the four divergent civilized disease pools, as he calls them, began to converge. The cause of this convergence was the increased trade between the Roman Empire, India, the Middle East, and China that was carried on by caravan and by ships plying the Indian Ocean. The author reasons that, in view of the great epidemics, perhaps smallpox and measles, which decimated the Roman and Chinese populations, infections must have been exchanged as well as goods. India and the Middle East, he states, seem to have escaped these epidemics. Interesting though these speculations are about the origin and nature of epidemics in Rome and China, they are largely just that in the absence of strong evidence.

McNeill is on somewhat firmer ground in discussing the advent of the bubonic plague in Europe in 542-543, during the reign of Emperor Justinian. He states that the bacillus of the plague, Pasteurella pestis, (isolated by disciples of Louis Pasteur only in 1894) was spread from an original focus in northeastern India-southwestern China or central Africa into the Mediterranean by ships that traversed the Indian Ocean and Red Sea. These ships were infested with the species of “black rats” which in turn were infested with the fleas that actually carried the bubonic bacillus into Mediterranean Europe. McNeill attributes the failure of Justinian’s efforts to restore imperial unity to the Mediterranean world in good measure to the decline of imperial resources resulting from the plague. Although this factor cannot be entirely discounted, a more realistic explanation for the failure of Justinian’s grandiose scheme of imperial reunification was the sheer enormity of the task whatever the status of his resources.

Europe’s most disastrous encounter, however, with the bubonic plague came in the period 1346-1350, when the “Black Death,” as it was called, swept over most of the Continent, killing about one-third of the population. McNeill holds that the establishment of the Mongol Empire between 1250 and 1350 with its incorporation of China, central Asia, and most of Russia led to the transfer of the Pasteurella pestis bacillus to the rodents of the Eurasian steppes for the first time. The infection must then have traveled the caravan routes of Asia before reaching the Crimea in 1346, where in that year it decimated the armies of a Mongol prince who, as a consequence, was compelled to break off the siege which he had laid to the Crimean trading city of Caffa. From there it was carried by the rather extensive shipping network of Europe to various ports on the Continent and then spread into the interior where it wreaked havoc on a population that had grown fairly dense by the fourteenth century. McNeill concedes that he is dealing in probabilities in showing how the bubonic plague eventually reached Europe at that time, but he is on stronger ground here than in his discussion of the spread of diseases in ancient times. The basis for his hypothesis is the documented pattern of the spread of bubonic plague in the nineteenth and twentieth centuries on a worldwide scale. McNeill points out that one of the results of the Black Death, aside from the great loss of life and its general disruption of feudal society, was that it discredited the rational theology of Thomas Aquinas in favor of a more mystical and personal relationship to God. While this view has some merit, it must be remembered that William of Ockham and Marsiglio of Padua had challenged Thomistic philosophy many years before the Black Death swept into Europe.

The author rounds out his discussion of the bubonic plague in Europe by offering several interesting hypotheses, which he labels as such, as to why in the years after 1346, some other diseases, such as leprosy (referring to the specific disease known today as Hansen’s disease) apparently declined while tuberculosis seems to have increased. One theory which McNeill advances is that of changing patterns of disease competition. In short, any immunity built up toward the more mobile tuberculosis bacillus may also have provoked an immunity to the slower moving leprosy bacillus once it arrived in the host. At best, however, this is only a theory.

In his analysis of disease exchanges, McNeill devotes considerable attention to the disastrous impact which the diseases of Europe and Africa had on the native inhabitants of the New World when abruptly introduced there during the Age of Discovery and Exploration. Little is known, the author relates, about the level of disease experience among pre-Columbian American Indians, except that it seems to have been quite low in comparison to that of the peoples of Eurasia. McNeill offers as many hypotheses to explain the apparent absence of infections among the civilized Indians of the New World as he does to explain their presence among the peoples of the Old. Hence, when the Spanish conquistadores arrived in Mexico and Peru in the sixteenth century, the smallpox they brought with them decimated those Indian nations which initially tried to resist their advance. This explains why Cortez and Pizarro, respectively, were able to conquer the Aztecs and Incas with only a handful of men. More significantly, these disasters, compounded in quick succession by outbreaks of measles, influenza, and perhaps typhus, further explain why the Spanish were so successful in transferring their culture and language to the New World. McNeill attributes the mass conversion of the Indians to Christianity to the fact that the Christian God seemed to spare the white newcomers from the ravishes of disease which he unmercifully visited upon the native populations because they were not of the Christian Faith.

Diseases of African origin, especially malaria and yellow fever, also took their toll of those Indians and some European settlers who inhabited the tropical regions of the New World. The mosquito carriers of the malarial parasite entered the New World beginning probably in the sixteenth century on ships bringing slaves from Africa. There, McNeill explains, malaria had kept natives in the tropics culturally backward through the centuries. In the mid-seventeenth century, the influx of yellow fever, transferred from West Africa, resulted in major epidemics which killed off large numbers of tropical Indian populations that had escaped decimation by malaria. Thus, according to McNeill, the introduction of these tropical infections from Africa in combination with the influx of deadly European diseases resulted in almost the total destruction of the pre-existing American Indian populations, especially those living in the tropics.

Plagues and Peoples is a synthesis based largely upon speculation, hypothesis, and conjecture. To a considerable extent, McNeill has taken the documented history of modern disease as a model to explain how infections developed and spread among and between peoples in ancient, medieval, and early modern times. Many of his observations on diseases, especially those in antiquity, are so speculative as to be of very questionable value. Despite this shortcoming, indeed perhaps because of it, the book should attain the author’s objective of providing what he refers to as “a target to shoot down.” McNeill bases his study mainly on secondary sources as contained in the fifty-five pages of chapter notes at the end of the book. Although the author’s style of writing is at times rather repetitive, Plagues and Peoples is a thoroughly engrossing, if controversial, book which provides the general reader with a good introduction to a world view of history by one of America’s leading historians.