Health officials are reporting an alarming increase in some dangerous superbugs at U.S. hospitals.

These superbugs from a common germ family have become extremely resistant to treatment with antibiotics. Only 10 years ago, such resistance was hardly ever seen in this group.

Infections from these superbugs are still uncommon. But in the first six months of last year, nearly 200 U.S. hospitals – about 4 percent – saw at least one case, the Centers for Disease Control and Prevent reported Tuesday.

“I would call them a major threat emerging in our hospitals,” said Dr. Arjun Srinivasan, an infectious disease expert at the CDC.

Health officials call them “nightmare bacteria” that have now been seen in 42 states and threaten to spread their resistance to more and more of their bacterial brethren.

“We only have a limited window of opportunity to stop spread” of these superbugs, said CDC Director Dr. Tom Frieden. At a press conference Tuesday, he said he was “sounding an alarm.”

The CDC urged hospital workers to watch for the infections and take steps to prevent passing the germs to other patients.

The report did not include better-known superbugs like the staph infection MRSA or the intestinal bug known as C-diff, which have plagued hospitals.

It focused on the superbugs that have emerged from one specific bacteria group. At least five of the 70 kinds in that family have developed resistance to a class of antibiotic called carbapenems – considered one of the last lines of defense against hard-to-treat bugs.

Some of those bacteria seem to have terrifying potential. Among them: Klebsiella pneumoniae, a bug that killed at least seven patients at a federal research hospital in Bethesda, Md.; and those made resistant by a gene called NDM-1, named for New Delhi.

The bacteria usually live harmlessly in the gut but can cause pneumonia, and urinary tract and bloodstream infections if they get into other parts of the bodies of patients with weakened immune systems. As many as half the patients who get the bloodstream infections die, Srinivasan said.

However, CDC did not provide figures on deaths attributed to these superbugs.

In 2001, U.S. hospitals reported that only 1 percent of samples from the bacterial family were resistant to the antibiotic carbapenems. By 2011, it had risen to 4 percent.

It was more of an issue in the nation’s 400 specialized, long-term hospitals – 18 percent of them reported seeing such a superbug. The Northeast had the most, followed by the South.

U.S. health officials are keeping a close eye on the NDM-1 superbugs, which first showed up in India in 2010 and have been seen as more of a concern in other parts of the world. Of the 30 cases in the U.S., about half have been reported since July, including eight patients at a Denver hospital.

(AP)