Lag Ba'omer is a minor Jewish holiday that traces back not to antiquity, but to the Middle Ages. Marked on the 18th day of the Jewish month of Iyar, the modern festivities include making a bonfire and roasting potatoes, franks, marshmallows and other fire-friendly foods on the flames. Few may realize it, but what we're celebrating is the cessation of a vicious plague that carried off tens of thousands of yeshiva...