The question of what killed Grand Duke Francesco I de’ Medici has been argued in court documents, dissected in forensic laboratories, and debated in academic journals for more than four centuries – and until 2026, nobody could say with certainty whether he was a victim of disease or his own brother.
Francesco I de’ Medici and his second wife, Bianca Cappello, died within hours of each other in October 1587. For more than four centuries, historians debated whether they succumbed to disease or were murdered with poison as part of a dynastic power struggle. The rumors were not idle. They had names, motives, and a suspect standing right there in the room.
A joint team from Yale University and the University of Pisa has ended that debate with molecular evidence. The study, published in the journal iScience in July 2026, marks the first time any scientist has genetically confirmed the cause of death for either Medici. Four hundred and forty years. One rib bone. One parasite.
A Death That Looked Like Murder
Francesco fell ill after visiting the Medici family’s villa in Poggio with his wife, Bianca Cappello. The estate stood near swampy rice fields where mosquitoes thrived. The couple died on back-to-back days after suffering repeated fevers. Court doctors described recurring fevers known in central Italy as “febbre terzana” – the local name for malaria.
Rumors of an assassination spread immediately, pointing to Francesco’s younger brother and rival, Ferdinando, as the perpetrator. Ferdinando was next in line to the throne, but he was at risk of being passed over in favor of Francesco’s illegitimate son, Antonio. Ferdinando had also visited the grand duke and his wife at their residence just before they fell ill.
The suspect, in other words, was standing right outside the bedroom door with a motive, a timeline, and a reputation. He took charge of his brother’s illness, compiling medical bulletins and minimizing the gravity of his brother’s condition in dispatches to the Vatican. After the deaths, he ordered immediate autopsies – an unusual step, apparently taken to protect himself from future accusations.
The Case That Wouldn’t Stay Closed
Since 2004, when exhumation and analysis of skeletal remains began for 49 Medici family tombs as part of the Medici Project, various studies had confirmed malaria as the cause of Francesco’s demise. However, a 2006 toxicological investigation determined that the couple were victims of arsenic poisoning.
Scientists examined remains believed to belong to Francesco and Bianca, and some chemical analyses reported elevated arsenic levels. The problem with elevated arsenic in Renaissance-era remains, historians noted, is that arsenic was also widely used as medicine at the time – meaning its presence in bones doesn’t automatically mean someone slipped it into a dish of food.
Donatella Lippi, a professor of the history of medicine at the University of Florence, coauthored the 2006 study that argued arsenic poisoning better explained Francesco’s final days. She pointed to historical records describing symptoms such as skin eruptions, swelling, and severe pain – signs, in her interpretation, compatible with acute arsenic poisoning. She has not changed her position following the 2026 DNA findings. “Contracting malaria does not mean dying from it,” Lippi wrote. “I believe Francesco I suffered from malaria, but he was poisoned and died of poison.”
What the Bones Actually Said
Serena Tucci, assistant professor of anthropology at Yale’s Faculty of Arts and Sciences, led a genetic analysis of the skeletal remains of Cardinal Giovanni de’ Medici and Grand Duke Francesco de’ Medici. Paleopathologists from the University of Pisa extracted DNA from four rib samples – three from Grand Duke Francesco and one from Cardinal Giovanni.
The researchers discovered molecular traces of P. falciparum and a second species, P. malariae, in the remains of Francesco de’ Medici. Plasmodium falciparum is the parasite responsible for the most lethal form of malaria in humans. Finding two species of it in the same set of bones is not ambiguous.
Valentina Giuffra, a professor of history of medicine at the University of Pisa and co-author of the study, explained that prior attempts to solve the mystery used paleo-immunological analysis, which had pointed to malaria but could not be considered definitive. According to CNN’s reporting, Giuffra noted that only ancient DNA could give an answer with a high degree of certainty. “DNA is certain. It solves the problem and the doubts. I think this is a definitive answer.”
Giovanni’s Hidden Surprise
In 1562, Cardinal Giovanni de’ Medici died of malaria – and twenty-five years later, his older brother, Grand Duke Francesco, succumbed to the same disease. Giovanni was only 19 years old. He had traveled to the Tuscan coast with his mother, Eleonora of Toledo, and his younger brother Garzia. The marshes in the area were full of mosquitoes carrying malaria. All three developed recurring fevers and died within a month.
Researchers found a novel strain of Plasmodium falciparum in the bones of Giovanni de’ Medici. This was a strain nobody had seen before. The strain includes two unique genetic mutations that likely derived from demographic expansion as the parasite spread across Europe.
When researchers compared the sample with other ancient malaria genomes dating from the Iron Age through the 1940s, they found close links with samples from Europe, Taiwan, and the Caribbean. Their results suggest this strain spread during a period of population growth in Europe. The Medici brothers didn’t just give researchers a cold-case verdict. They gave them a window into how a parasite moved across an entire continent.
Why This Matters Beyond the Renaissance
The Yale and University of Pisa team uncovered a previously unknown strain of the world’s deadliest malaria parasite, one whose genetic mutations sit squarely within an ongoing scientific controversy about how Plasmodium falciparum evolves to outpace modern drugs.
Malaria has not gone anywhere. The World Health Organization estimates that about 282 million cases of malaria caused 610,000 deaths in 2024 alone. The disease was eradicated in central Italy during the 20th century, but across sub-Saharan Africa, Southeast Asia, and parts of the Americas, it remains one of the most relentless killers of children under five.
Adalgisa Caccone, a senior research scientist in Yale’s Department of Ecology and Evolutionary Biology and co-author of the study, noted that the work generates data that can inform current and future research on malaria, which remains a deadly disease afflicting millions of people worldwide.
Alexander Ochoa, first author and a research scientist at Yale’s Department of Ecology and Evolutionary Biology, put the stakes plainly: the study of ancient DNA offers an opportunity not only to diagnose malaria in historical remains, but to build a picture of how the pathogen has changed its genetic profile across centuries – data that can directly inform how scientists track and respond to drug-resistant strains today.
How Science Reads a Dead Man’s Rib
Ancient DNA degrades. Bone exposed to centuries of humidity, temperature fluctuation, and microbiological activity becomes an almost impossibly difficult material to sequence. Prior immunological analyses performed by the University of Pisa team had indicated that both Giovanni and Francesco carried P. falciparum. But until now, no genetic assessment had been made on their skeletal remains to confirm those findings.
Getting from “the immunological data suggests” to “the genome confirms” required pulling parasite DNA out of bone fragments that are approaching 500 years old. The work was conducted jointly by Yale’s Department of Ecology and Evolutionary Biology and the Division of Paleopathology at the University of Pisa – two disciplines that don’t typically end up in the same sentence, let alone the same laboratory.
The researchers were careful not to oversell the result. The senior author acknowledged that arsenic cannot be completely ruled out as a contributing factor. Researchers can confidently say Francesco carried malaria parasites. They cannot say with complete certainty that someone did not also administer poison. If both happened, modern science may never be able to distinguish which ultimately caused his death.
What the Medici Case Gets Right About How We Remember Death
Part of why the Francesco I de’ Medici malaria question took this long to resolve is that the Medici family was the kind of family people expected to be murdered. Many members of the Florence-based dynasty, which presided over Italian banking and trade from the 15th to 18th centuries, died from malaria. But the dynasty’s dominance – the patronage, the political marriages, the banking empire, the papal connections – made it almost impossible to believe that something as ordinary as a mosquito had brought down its grand duke.
Arsenic was a Renaissance weapon of choice precisely because it was undetectable with the tools available at the time. The fact that Ferdinando was powerful enough to use it, motivated enough to want to, and present enough to have done it made the accusation feel inevitable. The doctors were right. The gossips were wrong. But the gossips’ version had a better story arc, and better story arcs survive longer than correct diagnoses do.
The 2026 DNA findings are not just a forensic verdict. They are a reminder that the most dramatic explanation for a death isn’t always the true one – and that 440 years of accumulated suspicion can be undone by a microscopic parasite that has been in a rib bone all along, waiting for someone to finally ask the right question with the right tools.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.