The artwork is considered a rare example of Late Gothic painting in Puglia and depicts St. Roch, a 14th-century French pilgrim who is said to have healed victims of the plague, and came down with it himself.
"Indeed, St. Roch is typically represented with a bubo on the upper thigh," paleopathologist Raffaele Gaeta, at the University of Pisa, Italy, told Live Science.
The painting in Bari, showing the saint as a bearded man with long, curly blonde hair, is no exception. The anonymous painter portrayed St. Roch with a swelling on the left thigh, made visible by a rolled-down sock.
"However, the altarpiece adds a new, realistic detail: a white, thin filament comes out of the lesion and almost reaches the knee," Gaeta said.
He noted that art historians wrongly identified this element as a long drop of pus emerging from the infected wound. [25 Grisly Archaeological Discoveries]
"We believe instead that the painter portrayed an ancient case of dracunculiasis, an infectious disease caused by a nematode worm, the Dracunculus medinensis, well known in antiquity," Gaeta and colleagues Fabrizio Bruschi and Valentina Giuffra wrote in their study.