Ring around the Rosy
One of the first visible signs of infection were red rings surrounding a rosy bump, all over the victim's body.
Pocket full of Posy
A common belief of the time was that the plague was borne on "foul air." The rationale was that people could protect themselves from the bad air by keeping their local air smelling sweet. That, and it also helped them deal with the smell of death...
On the other hand, another sign of infection was the foul stench that would begin to emanate from the victim's body as their lymph system began filling with blood. Those still mobile endeavored to mask their stench and avoid detection by carrying flowers on their person.
Ashes, Ashes,
In the terminal phases of the disease, victims would be hemorrhaging internally, sometimes triggering sneezing as it irritated the breathing passages. "Ashes" is a child's approximation of a paroxysm of sneezing. In this weakened state, a victim could, and often did, sneeze their lungs out. Messy...
We all Fall Down.
This is the Black Death nursery rhyme. Bad stuff.