Symbolism, Imagery, Allegory
While we do see some actual corpses in The Decameron (who can forget hilarious moments like Andreuccio in the tomb with the dead archbishop or Martellino being placed on top of the saintly dead Arrigo), the most important bodies are the ones we don't see. Boccaccio gives a vivid description of the volume, condition and treatment of dead bodies in the Introduction to the First Day, but for the most part, he leaves it at that.
And of course, he's right to do so. The brigata are looking to leave all of that behind as they indulge in the pleasures of a healthful locus amoenus. But those corpses are still out there, and the physicality of human existence can't help but creep back into the stories that are told by the crew. The intense sexuality of the stories and the extreme physical experiences of the characters all point back to an obsession with the body in its various states.
The dead body is a physical representation of human mortality. Okay, duh—everybody knows that. But it's also just one step in a process of human existence, and that leads us to the motif of the Ages of Man. In Boccaccio's day, this was a visual and conceptual scheme for thinking about human physical and psychological development, beginning with birth and ending with death (or resurrection). Each age/stage has its attributes and provides a shorthand way of defining a person or character.
The contrast between the corpses in the streets of Florence and the youth and freshness of the brigata suggest something more than just the inevitability of death. We're meant to think about the ever-changing nature of life—all the stuff that happens in between the happy sexual desires of youth and the gravity and permanence of death. It's not a coincidence, then, that these stages of life are arranged around a wheel when depicted in art, echoing the image of the Wheel of Fortune.