The overuse of antibiotics in medicine and agriculture has long been known to foster the emergence of germs that are resistant to drugs.

On Monday, the Centers for Disease Control and Prevention issued the first solid numbers on the extent of the problem. It said that at least 2 million Americans fall ill from antibiotic-resistant infections each year, of whom at least 23,000 die from the infections, a very conservative estimate.

The agency warned of “potentially catastrophic consequences” unless prompt action is taken. It said that up to half of the antibiotics prescribed for people are not needed or appropriately used (as when a broad spectrum antibiotic is used instead of a more targeted drug).

Overuse of antibiotics on farms, where they are often used to promote growth and prevent disease in healthy animals, also contributes to development of resistant strains of germs.

The new report, for the first time, puts 17 drug-resistant bacteria and a dangerous fungus into three categories based on how big a threat they pose. Three were deemed “urgent threats,” including a bacterium, known as CRE, that is resistant to most drugs and kills a high percentage of people who become infected with it.

Though it is rare, causing 600 deaths a year, it has been identified in health facilities in 44 states. Further spread of the germ or transfer of its resistance genes to other germs could lead to a “nightmare scenario,” the agency said. Twelve drug-resistant strains, including such common germs as salmonella, tuberculosis and MRSA, were classified as “serious threats.”

Unless prompt action is taken to reduce overuse, track and prevent the spread of resistance, and develop new drugs, the CDC’s director, Thomas Frieden, warned, “the medicine cabinet may be empty for patients with life-threatening infections in the coming months and years.”