A stubborn bacteria is spreading in U.S. hospitals, which could prove deadly for patients with already-weakened immune systems. And now the Centers for Disease Control and Prevention are stepping in, calling for medical professionals across the country to buckle down with stringent preventative measures.

The lethal bacteria is called carbapenem-resistant enterobacteriaceae — CRE for short. Unlike other bacterium in the enterobacteriaceae family (which includes E. coli), CRE has somehow become resistant to a powerful line of antibiotics called carbapenems, which are some of the strongest drugs we have, and are often used as a last line of defense.

Because CRE is resistant to carbapenems, infected patients — many of whom already have weakened immune systems from other health woes — are left to be treated with colistin, an older and less potent antibiotic that can be toxic to the kidneys. And CRE boasts unusually high mortality rates, killing about half of the people who get the deadly infection in their bloodstream.

"CRE are a nightmare bacteria," said CDC Director Tom Friedman. "Our strongest antibiotics don't work, and patients are left with potentially untreatable infections."

One in 25 acute-care facilities reported at least one case of CRE last year, according to the CDC. While they're still not very common,  "CRE's ability to spread themselves and their resistance raises the concern that potentially untreatable infections could appear in otherwise healthy people."

How did the bug get so powerful in the first place? Over-prescription, apparently: Experts say that half of all antibiotic prescriptions are unnecessary. Now the CDC is calling on doctors, nurses, and medical professionals to take precautionary measures, including frequent hand-washing, and the removal of intravenous lines and catheters as early as possible to "reduce the risk of infection," says ABC News.

"The goal of the campaign is to get this thing under control right now," says Dr. Richard Beset, ABC News' chief health and medical editor, "before CRE has a chance to spread to more hospitals and out into the community."