Federal officials from the Centers for Disease Control are investigating the death of Malcolm Casadaban, an associate professor of molecular genetics and cell biology at the University of Chicago, who died September 13 of an infection probably related to the Yersinia pestis bacteria that he was studying. Y. pestis, better known as the likely agent of the Black Death, was cultured in large amounts from Casadaban’s blood, and no other pathogens were identified.

Plague deaths are extremely rare in the United States, with a few hundred deaths reported annually worldwide. All forms of the disease—bubonic, pneumatic, and septicemic (blood infection)—respond well to antibiotics easily obtained in developed nations. What makes this incident even more surprising is that Casadaban was working with a weakened strain of Y. pestis commonly used in laboratory work and occasionally employed as a vaccine. The bacteria should not have been able to kill him.

The New York Times reports that antibiotics have been given to approximately 100 people who may have been exposed to Casadaban, who could have spread the disease through the air if he had the pneumonic form (possibly in conjunction with septicemic plague, both forms more rare and deadly than the much more noticeable bubonic plague.) There’s no word as to whether researchers will now be required to take more stringent precautions when working with this strain, but it’s likely that Casadaban had some sort of preexisting vulnerability to the disease. No more cases have been reported, and since the illness spreads quickly and symptoms appear shortly after infection, further infection is highly unlikely.