The sages counted two hundred and forty-eight limbs in the human body — the same number, they noted, as the positive commandments of the Torah. A curse, they taught, enters and exits through every one of them.
Watch the numerology. When the Torah commands the banning of Jericho, it writes, "And the city shall be accursed" (Joshua 6:17). By gematria — the Jewish practice of summing the numerical value of Hebrew letters — the key word amounts to 248. When a person is placed under a cherem, an anathema, the ban spreads through every limb until the body itself is steeped in it.
But the Torah of mercy answers with the same number. Habakkuk prays, "In wrath remember mercy" (Habakkuk 3:2), and the Hebrew letters of that phrase are a transposition of the letters for "accursed," adding up again to 248. The same count that binds a curse also lets it go.
Rav Yosef added a proverb: "Hang an anathema on the tail of a dog, and the dog will still go on doing mischief." A curse is powerful, yes, but it does not reform a wicked nature. Only repentance reaches into the 248 limbs and empties out the shadow.
(From the 1901 Hebraic Literature anthology, drawing on Moed Katan 17a.)