Federal health officials reported Monday that at least two million Americans fall ill from antibiotic-resistant bacteria every year and that at least 23,000 die from those infections, putting a hard number on a growing public health threat. It was the first time that federal authorities quantified the effects of organisms that many antibiotics are powerless to fight.

The number of deaths is substantially lower than previous estimates, in part because researchers from the Centers for Disease Control and Prevention stripped out cases in which a drug-resistant infection was present but not necessarily the cause of death. Infectious disease doctors have long warned that antibiotic resistance — in which bacteria develop defenses against antibiotics used to kill them — threatens to return society to a time when people died from ordinary infections.

“They have come up with hard numbers where it has been only guesswork,” said Dr. Stuart B. Levy, a professor of microbiology at Tufts University and the president of the Alliance for the Prudent Use of Antibiotics. “This sets a baseline we can all believe in.”

In 2007, the C.D.C. estimated that about 100,000 people died every year of infections they developed while in hospitals. Most of those infections were believed to be resistant to some antibiotics, but not necessarily the most widely used ones. And it was unclear how many of the deaths were caused by the drug-resistant infections. Monday’s report quantifies that.

Dr. Steven L. Solomon, the director of the C.D.C.’s office of antimicrobial resistance, acknowledged that the report underestimated the numbers, but said that was by design. Researchers were instructed to be conservative and to base their calculations only on deaths that were a direct result of a drug-resistant bacterial infection.

“This is a floor,” Dr. Solomon said. “We wanted the cleanest number, the least subjective number.”

One point of contention has been the extent to which industrial-scale animal farming contributes to the problem of antibiotic-resistant infections in humans. The government has estimated that more than 70 percent of antibiotics in the United States are given to animals. Companies use them to prevent sickness when animals are packed together in ways that breed infection. They also use them to make animals grow faster, though federal authorities are trying to stop that.

The report said that “much of antibiotic use in animals is unnecessary and inappropriate and makes everyone less safe.” It also said that about half of antibiotic use in people is inappropriate.

The 114-page report counts infections from 17 drug-resistant bacteria and one fungus, pathogens that Dr. Solomon said caused an overwhelming majority of drug-resistant bacterial infections in the country. It drew on data from five disease-tracking systems, including a major count of bacterial infections reported in hospitals in 10 different areas across the country. The count of deaths was based on mathematical models — one for each resistant organism.

One particularly lethal type of drug-resistant bacteria, known as CRE, has become resistant to nearly all antibiotics on the market. It is still relatively rare, causing just 600 deaths a year, but researchers have identified it in health care facilities in 44 states.

“We are getting closer and closer to the cliff,” said Dr. Michael Bell, a C.D.C. official who presented the data.

Infections from one of the most pervasive types of drug-resistant bacteria tracked in the report, MRSA, have been declining. Invasive MRSA infections in hospitals went down by more than half from 2005 to 2011, according to a paper published Monday in the journal JAMA Internal Medicine. However, the number of invasive MRSA infections picked up outside health care settings has not changed much, and researchers pointed out that the number of those types of infections has for the first time outstripped the number acquired in hospitals.

“This is a really big switch,” said Eli Perencevich, an infectious disease epidemiologist at the University of Iowa.

Hospitals have taken steps to prevent drug-resistant infections, but less is known about preventing infections outside hospitals, clinics and nursing homes. There has been little research, for example, on the extent to which antibiotic use on industrial farms contributes to resistant germs in people. The farm animal industry has been reluctant to allow data collection, and very few studies of the link have been done.

In a separate study in JAMA Internal Medicine on Monday, researchers from the Johns Hopkins Bloomberg School of Public Health sifted through health records of more than 440,000 people in the Geisinger Health System, which operates a network of clinics and hospitals in Pennsylvania, and found that patients who lived near farms and areas where manure was dumped were 38 percent more likely to develop a MRSA infection, particularly of soft tissue or skin, researchers found.

Some researchers not involved in the study said it offered evidence that living near farms increased one’s chances of developing a MRSA infection. But the study did not test farm animals or soil for MRSA, nor did it find evidence of the type of MRSA that is typically associated with farm animals, and others said the infections might have been caused by something other than antibiotics on farms.