Did malaria, not poison, kill the Medici brothers? New DNA evidence answers a 400-year-old mystery

For more than 400 years, the rumor persisted that Francesco de Medici was murdered. In 1587, the Grand Duke of Tuscany and his wife died suddenly; rumor had it that his own brother had poisoned him. Now, scientists have the receipts, and they point to something much smaller than a rival with a vial of arsenic.

Working with paleopathologists at the University of Pisa, Yale researchers analyzed DNA from the ribs of two Medici brothers, according to a Yale News report on the study. Malaria killed both of them, not poison.

Two brothers, one disease, 25 years apart
So the Medici family, in essence, ran Renaissance Florence. They had banking and political power in Tuscany for generations. Cardinal Giovanni de Medici died at the age of only 19 in 1562. His elder brother, the Grand Duke Francesco, died 25 years later, in 1587. The two brothers lie buried in the Medici Chapels at the Basilica di San Lorenzo in Florence, where most of the family are buried.


The researchers screened for Plasmodium, the genus of parasites responsible for malaria, in DNA extracted from three rib samples belonging to Francesco and one from Giovanni, according to the study ‘Ancient DNA analyses of remains of the Medici family (16th century) provide insights into the genetic variation of Plasmodium falciparum,’ published in iScience. They discovered a new strain of the deadly malaria, Plasmodium falciparum, in Giovanni’s bones. In the remains of Francesco, they found traces of that same species and a second one called P. malariae. The paper also says the Giovanni strain carried two unique genetic mutations, likely linked to the parasite’s spread across Europe, while Francesco’s bones held traces of both P. falciparum and P. malariae. Using ancient DNA from four rib samples, the team says the results help map how malaria evolved in Central Italy during the Renaissance.

That second finding is more important than it sounds. The fact that two species of malaria are found together is consistent with earlier findings from Belgium from the same time period, but more genetic sequencing is required before we can be certain that the two species were actively circulating together in 16th century Italy, said study first author Alexander Ochoa.

The Yale report adds that Francesco’s remains yielded molecular traces of both P. falciparum and P. malariae, and that the newly recovered Giovanni strain contained two unique mutations. The authors say the discovery helps track malaria’s spread and evolution in Central Italy, but they also caution that more sequencing is needed before co-circulation in 16th-century Italy can be confirmed

Why a swamp trip turned deadly
The locations help explain the story. In 1562, while on a family visit to the Tuscan coast, Cardinal Giovanni died of a disease contracted in a swampy area that was perfect for the breeding of mosquitoes. He fell sick alongside his mother, Eleonora of Toledo, and his younger brother Garzia. According to original iScience findings reported by Yale News, all three had recurring fevers that killed them within a month.

The paper ties the outbreak to the Tuscan coast, where marshes were known mosquito breeding grounds, and says the three victims died over the course of a month. It also notes that court physicians’ records described “febbre terzana” and that the treatments included bloodletting, a common practice that likely did more harm than good.

Francesco's death was much the same. He and his wife, Bianca Cappello, were living in the family villa at Poggio, surrounded by swampy rice fields alive with mosquitoes. Both developed fevers that came and went, a textbook symptom of malaria, and the couple died on consecutive days.

Indeed, the brothers were correctly diagnosed by the court physicians of the time, who noted a pattern of recurring fever that was well known to doctors of the day, called “febbre terzana.” Bloodletting was their favourite treatment, which researchers now say probably did more harm than good.

So why did the poisoning rumor stick around
Renaissance Florence was suspicious of Francesco and his wife dying at the same time, and Francesco's brother and rival, Cardinal Ferdinando de Medici, stood to gain the most from Francesco's death. That was enough for centuries of speculation that Ferdinando had them poisoned with arsenic.

Study co-author Valentina Giuffra, a history of medicine professor at the University of Pisa, put it plainly. According to Yale News, she said the genetic analysis supports both the historical medical records and previous research, and scientists can now say with greater confidence that malaria, not poisoning, killed the Grand Duke.

The study backs the old court records with genetics, showing that Francesco’s bones carried malaria, not poison. It also strengthens the case for the historical diagnosis by matching the molecular evidence with the fever patterns described in archival sources, closing off centuries of arsenic speculation.

This is not the first time researchers have tried to answer this question. Previous immunological testing on the same remains had already suggested P. falciparum in both brothers, but no one had performed a full genetic analysis on their skeletons until this study, Yale News reported.

What this means beyond a 16th-century whodunit
This research is not only for the history buffs. The 20th-century eradication campaigns eliminated malaria from central Italy, but it continues to be a major health problem worldwide today. In 2024, the World Health Organization estimates there were about 282 million cases of malaria worldwide and about 610,000 deaths. The vast majority of those deaths are in children under five.

WHO says malaria remains preventable and curable, but still spreads through infected female Anopheles mosquitoes and can become life-threatening fast: untreated P. falciparum malaria may progress to severe illness and death within 24 hours. The agency also notes that children under five, pregnant women, and travellers are at higher risk of severe infection.

By studying how the parasite evolved and spread through Europe centuries ago, scientists are aided in their work today. The falciparum strain recovered from Giovanni's bones has two mutations that may have arisen as the parasite spread across Europe, Yale News reports. Ancient DNA methods have come so far that we can now reconstruct the history of a pathogen that is still killing people, said senior author Serena Tucci, an assistant professor of anthropology at Yale. Co-author Adalgisa Caccone, a senior research scientist at Yale, added that the data doesn't just explain the past; it can also inform current and future malaria research.

Sometimes the biggest breakthroughs in medical science come from the last place you'd ever expect: a 450-year-old tomb in Florence, a handful of rib bones, and a mystery that finally got its ending.