According to Martin Gray:

 
Nursery rhyme. Simple, often nonsensical verse sung to very young children by their parents. Part of the ORAL tradition, many are very old. One famous one, 'Ring-a-ring-o, Roses', refers to the plague which swept through Europe in the sixteenth century: the roses (spots) and the sneezing were symptoms. Mother Gooses Melody (c 1765) is a famous early collection. Much research has been done on the meaning and origin of the rhymes by Iona and Peter Opie in The Oxford Dictionary of Nursery Rhymes (1951).