Something that's untainted has not been spoiled or contaminated. After a flood, a homeowner is lucky to find papers and photos that are untainted by the mildew that covers most of his things.
While untainted can describe things that are physically unspoiled or free from contaminants, it's more common to use the adjective to talk about the character of a person or group. A candidate with an untainted reputation has no record of controversy, and a town that's untainted by chain stores might have a more charming, old-fashioned character. The moral sense of the word — "pure," in a religious way — was the earliest meaning.