Ring around the pustulant oozing sore...

Ring around the rosey
A pocketful of posies
Ashes, ashes
We all fall down

Many scholars believe that this rhyme is somehow connected to the Great Plague of London in 1665, or perhaps earlier outbreaks of bubonic plague. This is entirely unsupported by textual sources, as there is no mention of the verse, nor written evidence of its existence, before 1881.

For those that believe that the rhyme describes the plague, each line is a clear analogy. The first line illustrates the round red rash that would break out on the skin of plague victims. The second line refers to scented sachets victims would keep in their pockets to cover the odor of their sores and the dying. Others believed that such sachets would purify the air and keep them safe from the plague. Yet another hypothesis is that the modern word “posies” is derived from an Old English word for “pus”, referring to a plague victim's oozing open sores. The third line would refer to when people - alive and dead - were gathered up into piles and lit on fire in a belief that burning the diseased bodies would not allow the disease to spread. The last line is most often believed to mean “we will all die.”
Source: Wikipedia