The country faces an infectious disease catastrophe caused by drug-resistant microbes, which affect an estimated 2 million people annually and kill 23,000 of them, federal health officials said Monday.

The new estimates mark the first time the Centers for Disease Control and Prevention quantified the pervasiveness of infections caused by drug-resistant strains and the number of people who die from them.

"If we're not careful, the medicine chest will be empty," said the CDC's director, Dr. Thomas Frieden, adding that, "without urgent action now, more patients will be thrust back to a time before we had effective drugs."

This is the second time in recent months Frieden has sounded an alarm, signaling a possible end to the era of miracle drugs.

In March, he pointed to the growing prevalence of carbapenem-resistant Enterobacteriaceae -- CRE -- in hospitals. These pathogens are resistant to the antibiotics of last resort. He calls them "nightmare bacteria."

Monday, he referred to "potentially catastrophic consequences" as he revealed a report during a news briefing containing a rogues gallery of 18 drug-resistant microbes. The pathogens pose a growing threat to public health.

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One, Clostridium difficile -- C. diff -- accounts for 14,000 of the 23,000 deaths. C. diff has been a formidable foe in Long Island hospitals. In New York, about 16,000 cases are reported statewide annually.

The new mortality figure, Frieden said, is a wake-up call. But some experts thought the death toll might be higher.

"It seems like a conservative number, not that 23,000 is anything to be cavalier about," said Dr. Bruce Farber, chief of infectious diseases at North Shore University Hospital in Manhasset. "We are working on antibiotic stewardship in the hospital, which is to police and monitor antibiotic usage.

"We went from 'Why not give an antibiotic just in case,' which was pervasive five to 10 years ago, to 'Do we really need to give an antibiotic,' and save these vitally important drugs."

Dr. Steven Solomon, director of the CDC's office of antimicrobial resistance, said the agency's researchers winnowed their statistics to reflect illnesses directly traceable to drug resistance.

Resistant pathogens, experts say, are capable of repelling one or more antimicrobial drugs.

Bacteria develop the capacity to resist medications when overexposed. Among the microbes listed in the report, many were multidrug-resistant. About half of all antibiotics prescribed are probably unnecessary, the CDC says.

The agency's figures were drawn from a 114-page report, which estimated the excess health care costs of drug resistance -- $20 billion annually.

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While many drug-resistant strains are contracted in hospitals, sources are widespread, researchers found.

For example, MRSA is largely spread in communities.

The report also noted that much of the agricultural use of antibiotics is probably unnecessary. Dosing animal feed with antibiotics, for example, is designed to encourage rapid growth in farm animals.

Estimates from other studies suggest about 80 percent of all antibiotics involve factory-farmed animals.

Farber and other experts cited the lack of new antibiotics in the pharmaceutical pipeline as a key reason for the crisis. Drug companies abandoned antibiotic development because more money was to be made in drugs for chronic diseases.

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"The easiest antibiotics to produce have already been discovered," said Dr. William Schaffner of the Infectious Diseases Society of America. The society is behind a major push to encourage the development of new antimicrobials.