It's not an old wive's tale: cranberry helps prevent and treat urinary tract infections. And it's not just the acidity: a compound in cranberries and blueberries keeps bacteria from sticking to bladder and urinary tract walls. Cranberries are high in several kinds of antioxidants, including proanthocyanidins, which give the ripe berries their vivid red color.
In the 1672 book New England Rarities Discovered, author John Josselyn described cranberries:
"Sauce for the Pilgrims, cranberry or bearberry, is a small trayling plant that grows in salt marshes that are overgrown with moss. The berries are of a pale yellow color, afterwards red, as big as a cherry, some perfectly round, others oval, all of them hollow with sower [sic] astringent taste; they are ripe in August and September. They are excellent against the Scurvy. They are also good to allay the fervor of hoof diseases. The Indians and English use them mush, boyling [sic] them with sugar for sauce to eat with their meat; and it is a delicate sauce, especially with roasted mutton. Some make tarts with them as with gooseberries."