Ring-a-ring o'roses
A pocketful of posies
A-tishoo ! A-tishoo
We all fall down.

From the BBC Bitesize education articles for 4-14 year olds :

Some people claim the nursery rhyme 'Ring-a-ring-o'-roses' is about the plague:

The 'roses' are the red blotches on the skin.

The 'posies' are the sweet-smelling flowers people carried to try to ward off the plague.

'Atishoo' refers to the sneezing fits of people with pneumonic plague.

'We all fall down' refers to people dying.

Others believe that it's just a nonsense rhyme. The fact that people are willing to believe that the nursery rhyme is about the plague shows how much importance it is still given today.