Tzaraat, the subject of this week's portion (and, in part, of last week's as well), has traditionally been translated as leprosy, a word seemingly derived from the Greek lepra (scales). Contemporary translators, reluctant to use the word, either substitute a euphemism like "skin blanch" (Robert Alter) or retain the Hebrew term (Everett Fox). In a parallel context, the World Health Organization has replaced "leprosy" with "Hansen's disease."

What lies behind this reticence? Biblical tzaraat simply doesn't meet the specifications of leprosy, defined by the National Institutes of Health as "an infectious disease characterized by disfiguring skin sores, nerve damage, and progressive debilitation." Not only, as Robert Alter notes, do the symptoms not correspond, but "there is scant evidence that leprosy was present in the Near East before the Hellenistic period."

Besides, if tzaraat were infectious, treatment would call for isolating the sufferer the instant he presented the first signs or symptoms. Yet the procedure delineated by the Torah allows for a postponement of the "diagnosis" for a week or even two, and the Talmud notes that prospective sufferers were not even examined during the major festivals when, arguably, the population density of Jerusalem—and the risk of contamination—was at its highest. Finally, whereas Hansen's disease is diagnosed by a dermatologist and treated with antibiotics, tzaraat was diagnosed by a priest and was not so much treated as "purified" by means of a ritual.

So if tzaraat was not leprosy, what was it? Aryeh Kaplan, a distinguished American Orthodox rabbi and scholar, defined it as "a physical symptom of a spiritual defect." What kind of defect was it; why was tzaraat an appropriately punitive sign of its presence; and how did the prescribed ritual ameliorate the condition?

One way to determine the etiology of the disease is by reference to individuals in the Bible who were afflicted by it. Among well-known personages, the first was none other than Moses, whose hand briefly turned "tzaraat as snow" at the burning bush (Exodus 4:7). Another was Moses' sister, Miriam, who was similarly stricken "tzaraat as snow" (Numbers 12:10) and spent a week in isolation. What did the two have in common?

It was not so much what they had as what they did: namely, according to talmudic tradition, engage in slander (l'shon hara, or evil tongue). Moses maligned the Israelites by suggesting they were likely to disavow his divine mission; Miriam impugned Moses' prophetic standing. Indeed, the Talmud not only views tzaraat as punishment for slander; it views it as retributive justice. As slander is the moral equivalent of murder, tzaraat is a figurative death penalty. A talmudic idiom designates slander as "blanching another's face in public"—an appropriate verbal metaphor for the physical appearance of tzaraat: as pale and as snow-white as death.

Similarly, just as slander offends the social order, tzaraat entails enforced social ostracism, enfeebling the offender while he contemplates his error. This, the commentators point out, is manifest in the ritual prescribed in the Torah for purifying tzaraat: "They shall take for the one being purified . . . a piece of cedar wood, a scarlet thread, and a clump of grass" (Leviticus 14:4). Rashi (1040-1105) explains: "Cedar wood: Because afflictions are caused by arrogance [the cedar being known for its loftiness]. Scarlet thread and grass: What is the antidote? He must humble himself like a thread and a clump of grass."

Jewish practice no longer marks out tzaraat as a disease—and no longer imposes isolation on slanderers. But the need is at least as great now as it was back then to take social responsibilities seriously and view interpersonal relations as, figuratively, a matter of life and death. Rabbi Abraham Isaac Kook is reported to have said that if the destruction of the Second Temple was due to gratuitous enmity and jealousy, the way to redemption lay through gratuitous friendship and affection—a timely thought for the approaching Jewish festival of freedom and redemption.

Moshe Sokolow, professor of Jewish education at the Azrieli Graduate School of Yeshiva University, is the author of Studies in the Weekly Parashah Based on the Lessons of Nehama Leibowitz (2008).