The largest U.S. outbreak on record of one particular strain of a so-called “nightmare bacteria” is fueling alarm among public health officials about the spread of potentially lethal drug-resistant infections.

The outbreak, which has been traced to Advocate Lutheran General Hospital in suburban Chicago, has so far infected 44 people, according to the Centers for Disease Control and Prevention. Since 2009, just 96 cases of the infection have been reported to the agency.

The bacteria strain, known as carbapenem-resistant enterobacteriaceae (CRE), is a form of superbug that lives in the gut and can carry a gene called NDM-1 that is resistant to practically all antibiotics on the market today. Perhaps more alarming, the gene can jump from bacteria to bacteria, making treatable infections untreatable.

Before the Illinois outbreak, the largest spread of NDM-producing CRE that officials had seen was eight cases at a Denver hospital in 2012. When signs of the infection surfaced in Illinois, officials contacted 243 patients they believed came into contact with the bug during an endoscopic procedure. Of the 114 who returned for testing, 28 screened positive as carriers but did not have any symptoms. Ten others showed symptoms. The CDC also found six additional cases at a separate Illinois facility.

The bug at the center of the Illinois outbreak is among those detailed in a 2013 report by the CDC warning of “potentially catastrophic consequences” if action isn’t taken against the threat of antibiotic-resistant infections. That threat, which kills an estimated 23,000 Americans each year, was the focus of the recent FRONTLINE investigation Hunting the Nightmare Bacteria. Below, watch the story of one of the first cases of NDM-1 to be identified in the U.S.: