In the 1340's the eruption of the bubonic plague decimated entire European continents as the Black Death Plague wiped out 25% to 50% of Europe's population. Italy was particularly hit hard in the larger cities with a lose of up to 60% of their population.
This affected art in several ways, most importantly in my opinion by the over all depression and darkness that took over the artist that remained. With death happening all around, a lot of artist started producing more ghastly and gruesome works of art. And really who could blame them, it would be hard to produce upbeat, happy works of art when everything seemed so hopeless and dismall around you.
Another effect of the plague, was the art world was stimulated by religious encouragement and devotional images. They were commisioned to do burial and other religious ceremonies with little input of their own. With some much death and dispear around them there was no work to be had, so they had to take the work that they could commision from the church to make any money at all.
The focus on sickness also led to a decline in artistic importance. More money and energy was being put into hospital construction and medical research due to the necessity to find a cure for the plague. The hospitals and churches were one and the same and they incouraged people to put all faith in the church. With so much focus on religion, art became a neglected luxury.