One of the most important customs of Passoverowhich celebrates the Israelites' exodus from slavery in ancient Egyptois the removal of grain, leavened bread, and even stray crumbs from the home. The replacement of bread with matzo, or unleavened bread, commemorates the haste with which the Israelites fledoso hurriedly that dough prepared for their journey had no time to rise.
Martin Blaser, an infectious-diseases physician at Vanderbilt University, has a different idea about how the tradition arose. He thinks that the removal each spring of bread and grain from Israelite homes may have protected them from a rat-borne scourge: Yersinia pestis, the bacterial cause of the plague infamous in medieval Europe as the black death.
Passover, says Blaser, most likely originated from a fusion of two ancient Middle Eastern spring festivals: the ceremony of the sacrificial, or paschal, lamb still practiced among desert nomads, and the Feast of Unleavened Bread, which celebrated the barley harvest. As nomads took up agriculture, says Blaser, they would have become more vulnerable to plague. Stored grain attracts rats, which harbor fleas that transmit Y. pestis bacteria. (Indeed, a 1993 outbreak of plague in the former Zaire was linked to the grain stockpiling that followed years of civil unrest.) But clearing grain from the home in the springoa peak period for plagueoforces rodents to search elsewhere for food.
If the Israelites celebrated the Feast of Unleavened Bread while in Egypt, they might have been protected when plague struck. And it's possible that it did strike and that they survived; the Bible describes ten plagues that preceded the Israelites' flight, the last of which killed Egyptian firstborns and cattle. Plague, says Blaser, is one of the few human diseases that attacks domesticated animals with a high degree of mortality.
There's no concrete evidence that Jews were spared when plague decimated Europe in the fourteenth century. In fact, they were accused of causing it and many were massacred. There are, however, reports that Jewish deaths were half those of Christians when plague struck Venice in the spring of 1631. If Blaser's theory is correct, it could explain why the annual removal of grain became enshrined in Jewish law. "If you had certain beliefs that you thought were very good for your community and wanted them to be around in perpetuity," he says, "then you would try to mix them very deeply into the belief system."