What disease does Ring Around the Rosies relate to?

No, but it's ABOUT a disease. It's about the black plague.

Ring around the rosies

-One of the first signs of the disease was a red bump,known as a rosy,surrounded by a red ring

Pocket full of posies

-The piles of dead bodies had a stench people COULD NOT BEAR, plus they were growing every day,so people carried little bags of nice smelling herbs,put them in their own pockets,and sprinkled them over the dead.

Ashes, ashes

-People don't say ACHOO when they sneeze. They say ASHES. Try it. It also refers to cremation because people were afraid the dead would turn into vampires or Zombies, and it got rid of them for good.

We all fall down

-We all fall down and die.