Doctors have long worried about the evolution of a disease that would defeat antibiotic treatments. There are now two such “superbugs” on the medical radar screen, and one of them is spreading through the United States.

The U.S. bacteria has been dubbed the “Phantom Menace,” cribbing the name of the first Star Wars prequel, because it has been difficult to detect. The Washington Post explains how it works:

This superbug’s strains belong to the family of bacteria known as CRE, which are difficult to treat because they are often resistant to most antibiotics. They are often deadly, too, in some instances killing up to 50 percent of patients who become infected, according to the CDC. Health officials have called CRE among the country’s most urgent public health threats.

The target of Thursday’s report is relatively new. Unlike more common types of CRE, it carries a plasmid, or mobile piece of DNA, with an enzyme that breaks down antibiotics. And what makes these bacteria even more dangerous is their ability to transfer that plasmid–and that antibiotic resistance–to normal bacteria that are present in our bodies.

“What we’re seeing is an assault by the microbes on the last bastion of antibiotics,” said Centers for Disease Control Director Thomas Frieden.

In essence, we have a bacteria spreading a contagion to other bacteria that makes them resistant to antibiotic treatment. The other strain described in the Washington Post report was discovered in China two weeks ago, and a case has been reported in Denmark.

This superbug is resistant to colistin, the “antibiotic of last resort.” The possibility that its cloistin-resistant gene could spread rapidly to other bacteria, around the world, is described by health experts as “frightening.” In November, the BBC’s report on the Chinese superbug said it could mark the beginning of the “post-antibiotic era,” more sensationally described as the “antibiotic apocalypse.”

The CDC hopes that new definitions for the CRE organisms will make diagnosis easier. Only 43 cases have been found between 2010 and 2015, but there are fears the accelerating rate of infections could lead to a breakout.

“Monitoring the emergence of carbapenemases is crucial to limiting their spread; identification of patients carrying carbapenemase-producing CRE should result in the institution of transmission-based precautions and enhanced environmental cleaning to prevent transmission,” the CDC declared in its December 4 field notes. According to this report, the patients so far have been elderly (the median age among the 35 patients with age reported was 70) and many of them had traveled or been hospitalized internationally, most commonly to India.