"Seven year itch" originally referred to scabies

The seven-year itch that [Thoreau] had in mind was an infection by a mite which lays its eggs in burrows under the skin. Its medical name is scabies, whose name comes appropriately from Latin scabere, to scratch. It was once extremely common in all kinds of situations and historical American sources are full of names for it, among them Indiana itch, Illinois itch, Jackson itch, Cuban itch, prairie itch, camp itch, army itch, ship itch, jail itch, mattress itch, swamp itch, winter itch, barley itch and grain itch. It was very hard to treat before effective insecticides came along...

Because it was so hard to get rid of, a story grew up in North America that those who got the itch were stuck with it for the next seven years. The phrase was sometimes later reinterpreted to mean that it would recur after seven years, or would reappear every year for seven years. Found at World Wide Words, where the discussion continues as to how and why the phrase was adapted as the title of the famous movie.