The strain that was supposed to be harmless.
Professor Malcolm Casadaban worked with a strain of plague the federal government had decided was safe. The strain, KIM D27, is missing the stretch of DNA that carries the genes Yersinia pestis needs to pull iron out of a host. No iron means no growth, so researchers treated it as unable to infect a person. On that reasoning, the CDC left it off the select agent list, the registry that says who may handle dangerous germs. The exclusion kept his work outside those rules. In September 2009, it killed him.
Professor Scott Minnich, a microbiologist at the University of Idaho, wrote to me about Casadaban on Friday, in the middle of a week of questions about Irkutsk. “I say this only because lab accidents do rarely occur,” he wrote. “Malcolm’s death hit the news but nothing like this present incident.”
What happened in that Chicago lab?
At the University of Chicago, Casadaban was an associate professor working with Dr. Olaf Schneewind on a plague vaccine. He did his PhD in Professor Jonathan Beckwith’s lab at Harvard, where he worked out how to fuse a gene to a marker so you could watch it switch on. Thousands of people have used the trick since without knowing whose it was. “I used it routinely,” Minnich wrote, “and it remains a powerful technique and a stellar contribution to molecular biology.”
He got sick around September 7, and saw a clinician three days later. He reached a Chicago emergency department on September 13th, by which time he’d been sick for six days. About 12 hours after he arrived, doctors intubated him, and he died an hour later. He was 60.
“Olaf Schneewind and I were having a beer at Cold Spring Harbor Laboratories after a long day of meetings,” Minnich wrote, “when he received the phone call delivering the news of Malcolm’s death and the unexpected plague diagnosis.”
What the tidy version leaves out.
Casadaban had hemochromatosis, which nobody knew, so his blood carried too much free iron and the crippled bacterium fed on it.
The CDC’s report calls iron overload “one possible explanation.” The hemochromatosis diagnosis came after he died, from tissue, labs and a genetic test. He also had insulin-dependent diabetes, which that version drops. Nobody ever established how the bacteria got into him. The pathology pointed away from his lungs and toward his skin or a mucous membrane, and the report notes that gloves weren’t worn consistently in his lab.
“Though the exact route of exposure remains uncertain,” Minnich wrote, “the incident sent deep concern throughout the Yersinia research community.”
In 2012, Casadaban’s colleagues tested whether the iron explained it. Lauriane Quenee, with Dr. Schneewind as senior author, gave strains like KIM D27 to mice bred to model hereditary hemochromatosis, and those mice developed lethal plague while normal mice did not. They recommended a different product: people with hereditary hemochromatosis “may be protected with subunit vaccines but should not be exposed to live-attenuated plague vaccines.”
Sixty-five contacts, thirty coworkers, no further cases.
The response never rested on the classification being right. Investigators found 1 household contact and 64 other close contacts, and 61 of them took antibiotics. All 30 of his coworkers got a prescription too. Nobody else got sick, although Science reports that the case still pushed the US to tighten its rules for weakened strains.
Pneumonia of undetermined etiology
Darya Shipilova was hospitalized on September 29th and died on October 2nd, at 28. She authored two research publications: an abstract on which antibiotics are effective against Yersinia pestis, and a 17-author monograph her institute released in 2024 on improving live weakened vaccines for plague, tularemia, and anthrax. Russia told WHO she had been vaccinated against the high-threat pathogens she worked with.
Pneumonic plague needs close, sustained contact. Across eight documented outbreaks between 1907 and 1997, one case infected an average of 1.3 others before public health measures began. Isolating the sick and watching their contacts stopped every one of those outbreaks but Mukden in 1946. For 7 days Russia watched her contacts, with daily testing, and reported no symptoms in any of them.
The plague vaccine Russia uses is a live weakened strain. Nobody has said whether Russia tests the people it vaccinates for hemochromatosis, or whether anyone tested Shipilova.
Nobody outside Russia can check any of it. WHO asked for verification on October 3rd, 5th, 6th and 7th under the International Health Regulations, and again about a rumored second case. Russia answered that it had run more than 5,500 laboratory tests, that the panel covered influenza, SARS-CoV-2, plague, and streptococcus, and that among the contacts it found two cases of COVID, three of rhinovirus, one of Haemophilus, and other opportunistic infections. It still calls her cause of death “pneumonia of undetermined etiology.”
On October 10th, WHO said that “a laboratory-associated exposure therefore cannot be fully excluded at this stage,” and that the agency “is not in a position to independently assess whether the duration of the reported monitoring period for contacts was sufficient to exclude all plausible causes of the fatal illness.” It asked Russia to continue testing her clinical specimens, with reference laboratory support if needed, and offered support on high-threat pathogens and laboratory biosafety.