Demons were not abstract theology for medieval Jews. They were a daily hazard requiring specific countermeasures, and undefined Trachtenberg catalogued an elaborate system of protections that governed everything from walking at night to using the bathroom.
The most basic rule: never travel alone after dark. The Talmud taught that two people walking together are safer but must stay alert; three together need not fear at all. A torch counted as one companion. Judah ben Bezalel of Prague elaborated in his commentary Derech Hayim that even the light of a candle could repel the spirits that roamed freely between sunset and dawn. The Friday night Amidah prayer was shortened specifically to get worshippers home before demons emerged—though as Professor Ginzberg noted, this was a later superstitious reinterpretation of what was originally just the only evening service of the week.
Certain people attracted demonic attention more than others. A bridegroom and bride were considered especially vulnerable, as were mourners, the sick, and women in labor. The Sefer Hasidim warned against sleeping alone in a house, pouring water at certain hours, and eating food left uncovered overnight—all entry points for demonic contamination.
The most dramatic form of demonic interference was possession. A dibbuk (דיבוק)—the spirit of a dead person that enters a living body—appears in Jewish literature from at least the 16th century, first attested in writings about undefined Luria and his students in Safed. A protocol from 1571 describes an actual exorcism. But the concept drew on much older beliefs about restless souls and their ability to cross the boundary between the living and the dead. The Kabbalists linked dibbukim to the doctrine of <i>gilgul</i> (reincarnation)—these were souls too damaged to find a new body through normal channels, forcing their way into an already-occupied one.