The English word “quarantine” is a direct descendent of quarantino, the Italian word for a 40-day period. Why 40 days? Health officials may have prescribed a 40-day quarantine because the number had great symbolic and religious significance to medieval Christians. When God flooded the Earth, it rained for 40 days and 40 nights, and Jesus fasted in the wilderness for 40 days. Stevens Crawshaw says that even before the arrival of the plague, the biblical notion of a 40-day period of purification had crossed over into health practices. After childbirth, for example, a new mother was expected to rest for...