This painting probably hung in a sanatorium, where people were dying from the plague.
This painting gave them hope. It is not about death, but about life. About conquering death.
St. Sebastian was supposed to die, but he recovered from near-death. He was proof to the plague sufferers that you could recover from near-death, and they prayed to him for intervention. And indeed, in the painting, you see St. Sebastian -at their request- talking to Jesus and begging for their recovery (Sacra Conversazione).
St. Roche suffered from the plague himself. From the staff and scallop shell you can see that he went on a pilgrimage to Satiago de Compostela (North-West Spain), to pray at the grave on apostle James, and after that then went a little further west to "the end of the world", the village of Finisterra. There, all pilgrims picked up a specific shell from the beach as proof that they had made the journey.
Although St. James suffered from the plague (almost a certain death sentence) he was cured. A dog befriended him and brought him food and licked his sores and the boils healed and he was fully cured. A great example and inspiration for the people, who prayed to the people in the painting. St. Roche is showing us his leg as proof that he is totally cured. No more sores or boils.
And the final proof that the plague is conquered is in the way the dragonfly is shown. The dragonfly was the symbol for the plague. At first glance it looks like the dragonfly is on the rock, but take a step back and visualize the rock as a block of amber, with the dragonfly encapsulated in it, and Mary and Jesus sitting on top of it. As conquerors. The plague has been "neutralized" and conquered on all accounts.
The final observation below is not from David Weeks, but by Willem van Osnabrugge from internet searches:
The "Black Plague" was the disease we call bubonic plague, spread by a bacillus usually carried by rodents and transmitted to humans by fleas. The plague first hit western Europe in 1347, and by 1350 it had killed nearly a third of the population. Every now and then the plague flared up again in Western Europe.
Sufferers had big red sores, with black rings around them (often called rosies).
A later bad outbreak of the plague, which occurred in London in 1665, resulted in the following nursery rhyme:
Ring around the rosies
The practice of carrying flowers and placing them around the infected person for protection is described in the phrase, "a pocket full of posies." "Ashes" is a corruption or imitation of the sneezing sounds made by the infected person. Finally, "we all fall down" describes the many dead resulting from the disease.
The "pocket full of posies" is also said to by any one of the
Likewise, multiple meanings are claimed for the repetition of
"ashes" at the beginning of the last line:
So, next time when you hear your (grand)children sing this song, you'll never feel the same way again.