CHICAGO (Reuters) - Public health officials are investigating the death of a University of Chicago researcher who studied plague bacteria and was found to have the microbe in his blood, university officials said on Monday.

Malcolm Casadaban, who died on September 13, was researching a weakened strain of the plague bacteria Yersinia pestis. Because it is missing key proteins, the strain is not normally harmful to people.

Medical center spokesman John Easton said Casadaban had the laboratory strain of Yersinia pestis in his blood, suggesting he had a form of the infection known as septicemic plague, which can kill even before the symptoms begin.

“This organism has been studied in the lab for 40-plus years. This is the first time this has happened,” Kenneth Alexander, an infectious diseases expert at the University of Chicago Medical Center, said in a telephone interview.

Public health officials have tracked down about 100 people who had contact with the researcher, and are giving them antibiotics as a precaution, but said there is no public health threat.

“Nobody else is getting sick. The incubation period is two to 10 days. We’re already at the outer limits of when it should rear it’s head if it were there,” Alexander said.

He said investigators plan to study the genetic sequence of the bacterium in the scientist’s blood to see if it had somehow mutated, although early tests suggest it had not.

The most likely explanation, Alexander said, is that the researcher had an underlying genetic susceptibility to the bacterium, such as an excess of iron in the blood, that may have allowed the infection.

There are about 20 cases of plague in the United States each year, mostly in parts of California, Colorado, Utah, Arizona, Nevada and New Mexico. It is carried by fleas that live on rodents such as prairie dogs and rats.

The last known case of person-to-person transmission of plague in the United States was in 1924.

Plague is still a problem in the developing world, where about 3,000 cases are reported each year.