The researcher, who died Sunday, studied the genetics of harmful bacteria, including a weakened strain of yersinia pestis.
The University of Chicago says there doesn't appear to be any threat to the public, and no other illness related to the case has been reported.
The weakened strain does not require the safety precautions associated with working on virulent strains. The modified strain of "y-pestis" has been approved by the Centers for Disease Control for routine laboratory studies, and it is not known to cause illness in healthy adults.
The departments of public health for both Chicago and the state, as well as the CDC, are investigating.