Since after the Second World War, the rhyme has often been associated with the Great Plague which happened in England in 1665, or with earlier outbreaks of the Black Death in England. Interpreters of the rhyme before World War II make no mention of this; by 1951, however, it seems to have become well established as an explanation for the form of the rhyme that had become standard in the United Kingdom. Peter and Iona Opie, the leading authorities on nursery rhymes, remarked: