In all innocence, children have for centuries sung a nursery rhyme that is in truth anything but an innocent verse:

Ring-a-ring o’roses,
A pocket full of posies,
A-tishoo! A-tishoo!
We all fall down!

The rhyme arose about 1665 in the streets of London during a plague epidemic of the Black Death. Each phrase of the rhyme refers to an aspect of the plague.

“Ring o’roses” is a reference to t…