The largest outbreak to date of one strain of what authorities have called "nightmare bacteria" is adding to concerns about the spread of such drug-resistant bugs.

The outbreak, centered on a hospital in a Chicago suburb, has infected 44 people in Illinois over the past year, the Centers for Disease Control and Prevention said. The bug, known as carbapenem-resistant enterobacteriaceae, bears a rare enzyme that breaks down antibiotics.

"This is a huge cluster," said Alex Kallen, a medical officer with the CDC and supervisor for the Illinois outbreak investigation, noting that only 97 cases of the infection have been reported to the agency since 2009.

The superbug, which typically lives in the intestines and is part of a large family of bacteria, can be spread via fecal matter. It isn't typically transmitted by casual contact outside hospitals.

Click for more from The Wall Street Journal.