tl;dr; - Scientists did DNA testing on the remains of the Grand Duke and his wife, and found malaria. The rival theory is poisoning by the relative who was next in line for the Ducal throne. HOWEVER, this test result solves nothing:
But is there any guarantee that Francesco was not also poisoned?
“Perhaps not,” Ochoa said, “but the genetic evidence presented in our study decreases the margin for speculation.”
Gisella Caccone, a senior research scientist also at Yale and a study coauthor, agrees. “We can say that they had malaria, we cannot say that they were not poisoned as well,” Caccone said in an email.