"Nightmare bacteria" that have become increasingly resistant to even the strongest antibiotics infected patients in 3.9 percent of all U.S. hospitals in the first half of 2012, including 17.8 percent of specialty hospitals, public health officials said on Tuesday.

"Our strongest antibiotics don't work and patients are left with potentially untreatable infections," Dr Tom Frieden, director of the Centers for Disease Control and Prevention, said in a statement. He said doctors, hospitals and public health officials must work together to "stop these infections from spreading."

At a news conference, he added, "it's not often that our scientists come to me and say we have to sound the alarm, but that's what we are doing today."

Over the past decade more and more hospitalized patients have been incurably infected with the bugs, Carbapenem-Resistant Enterobacteriaceae (CRE), which kill up to half of patients who get bloodstream infections from them, according to a new CDC report. The report did not say how many patients were killed by the bacteria.

Enterobacteriaceae bacteria include more than 70 species that normally live in the water, soil and human digestive system, such as the well-known E. coli. Over the years, some Enterobacteriaceae have become resistant to all or almost all antibiotics, including last-resort drugs known as carbapenems. Present in one U.S. state in 2001, the superbugs have now spread to 42, Frieden said at the news conference.

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