Bold, novel theories, sure to be controversial, about the medieval pandemic known as the Black Death, by late Brown University historian Herlihy. The European pestilence (dubbed the Black Death centuries later by northern European scholars) began in 1348 and ravaged the continent in intermittent waves for a century. In that time it killed millions; Herlihy estimates that in villages as far apart as England and Italy populations were reduced by as much as 70 or 80 percent. It is regarded as one of European history's watershed events. While not disputing that, Herlihy revisits much of the conventional wisdom about the demographic, cultural, and even medical impact of the plague. Indeed, he questions whether the Black Death even was plague: He notes that medieval chroniclers did not mention epizootics (mass deaths of rats or other rodents, which are a necessary precursor to plague) and did mention lenticulae or pustules or boils over the victims' bodies, which is not characteristic of plague. Herlihy observes that the illness showed some signs of bubonic plague, some of anthrax, and some of tuberculosis, and speculates that perhaps several diseases ``sometimes worked together synergistically to produce the staggering mortalities.'' Herlihy sees Europe before the Black Death as engaged in a ``Malthusian deadlock'' in which a stable population devoted most of its energy to production of food and subsistence goods. The precipitous population decline occasioned by the Black Death compelled Europe to devise labor-saving technologies that transformed the economy. In more controversial theories, Herlihy argues from the increased use of Christian given names that the Black Death caused the Christianization of what had formerly been a pagan society with a Christian veneer, and contends that in the wake of the pestilence Europeans turned to preventive measures such as birth control to check explosive population growth. A stimulating discussion of some rarely considered aspects of one of history's turning points.