One of my favorite discoveries as a graduate student was a book entitled Rats, Lice, and History, by Hans Zinsser—a classic, early treatment of the episodes in which medieval Europe periodically was swept by disease, decimating the population, remaking the social and political fabric, and arguably putting history on a different course. A particularly telling point that I remember still was that, despite desperate efforts by authorities and scientific thinkers of the time to grasp the causes of these scourges and somehow to forestall their recurrence, solid understanding eluded them. The primitive public health policy of the day fell back on a few crude rules (supported by myth and prayer), and prevention was not achieved. In fact, echoes of the original plagues typically recurred at intervals until some version of natural immunity emerged in the surviving population, mainly through the harsh combination of exposure and attrition. For those concerned with financial market crises, obvious sobering analogies to market processes come to mind here.