Just as incurable viruses gain new footholds around the world, a growing number of bacterial infections that were once easily treatable are now withstanding modern medicine’s arsenal of antibiotics. Twenty-three thousand Americans die from antibiotic-resistant pathogens every year. Methicillin-resistant Staphylococcus aureus, or MRSA, is among the most notorious. The number of adults hospitalized due to another culprit, a resistant strain of Clostridium difficile has nearly doubled over the last decade, according to a study published last week.

Largely to blame for this public health predicament, experts say, is the continued misuse and overuse of antibiotics in both humans and animals. A case in point: The first person diagnosed with the Ebola virus in the U.S. was initially sent home with antibiotics. The drugs, of course, wield no power against viruses.

But it’s their use in animals that has sparked the loudest debate. Despite warnings going back to penicillin-discoverer Alexander Fleming in the 1940s and the U.S. Food and Drug Administration in the 1970s, as well as successful efforts over the last decade in Denmark, the Netherlands and other European countries to curb the practice, livestock producers across the U.S. continue to routinely feed healthy animals small doses of antibiotics. Some of the antibiotics belong to the same classes of drugs used to treat people — including key weapons in fighting urinary tract infections and infections after surgeries. Even use of antibiotics not considered important in human medicine, experts now warn, can promote cross-resistance to the more critical drugs.

“The overwhelming proportion of antibiotics are used in animal feed in a very uncontrolled fashion,” said Ellen Silbergeld, a professor at Johns Hopkins School of Public Health. “It is the perfect recipe for creating antibiotic resistance.”

Overall, cattle, swine, chickens and other livestock receive about 80 percent of the nation’s antibiotics — with most of those drugs administered in low concentrations to prevent the spread of disease or simply to promote growth. Just as an incomplete course of antibiotics can result in the rise of a more virulent infection in a person, this sublethal use in animals means bacteria that can withstand the drugs will survive, reproduce and pass on their resistance to the next generation of bugs on the farm. In the end, animal antibiotics are thought to affect human health via multiple pathways: direct or indirect contact with food, water, air or anywhere urine or manure goes.

A study published on Tuesday builds on evidence that antibiotic residue in the environment spurs the growth of resistant bacteria — at even lower concentrations than previously thought.

Last week the FDA released some relevant new data: It reported that the total quantity of antibiotics that are known to be important in human medicine and are sold or distributed for use in food-producing animals jumped by 16 percent between 2009 and 2012. In total, 61 percent of the antibiotics sold to the meat industry are considered medically important.

The Animal Health Institute, which represents pharmaceutical companies, suggested the new data tell a “small part of the story.” Recent reports, the industry group told The Huffington Post in a statement, support their position: Just because food animals consume the bulk of antibiotics, responsibility for the majority of the human health problem doesn’t necessarily rest on the industry.

A report published in September by the President’s Council of Advisors on Science and Technology also puts a greater emphasis on human health contributions — although it does raise concerns over a role for animal agriculture, and touts the FDA’s new voluntary guidance on the use of the drugs in animals. The agency has asked drug companies to voluntarily change their labels by December 2016 to exclude uses for growth promotion. If a label changes, then farmers or feed mills would need to obtain a prescription from a veterinarian to treat a sick animal or to prevent disease. Before, they could simply buy the drugs over the counter and administer them without any involvement of a veterinarian.

Silbergeld criticized the PCAST report for not putting the animal agriculture issue “front and center,” and argued that the FDA’s voluntary policy doesn’t go far enough. She is also among critics who point to a potential loophole in the strategy, as illustrated by experiences in Europe.

Dr. Jan Kluytmans, a professor of microbiology and infection control at University Medical Center Utrecht in the Netherlands, recalled the initial difficulty when his country began trimming use of antibiotics in their livestock.

At the turn of the century, he told HuffPost, animals were receiving approximately 95 percent of the country’s antibiotics. And as in the U.S., most of that medicine was not given to sick livestock. So in 2006, the government banned the use of antibiotics as growth promoters and began requiring veterinarian oversight for other uses.

“But that didn’t result in any reduction in use of antibiotics in animals,” Kluytmans said. Rather, use of the drugs as therapy increased dramatically.

“When the last few drugs were banned for growth-promoting purposes, they did what we fear is going to happen in the U.S.,” said Laura Rogers, project director for the Pew Campaign on Human Health and Industrial Farming. “They called everything else a therapeutic use. They used tetracycline like it was candy.”

It wasn’t until the country faced two subsequent outbreaks of antibiotic-resistant infections, whose sources were traced to animals, that the numbers began to fall. “There was a lot of public awareness and media attention,” said Kluytmans. “Farmers and others involved realized that they could not continue in the same way.”

Since 2009, use of antibiotics on Dutch farms has dropped by about 60 percent. An independent authority now tracks antibiotic usage on each farm. What’s more, Kluytmans said, there have been no “measurable negative effects” on the animals or on productivity.

Obtaining such detailed information — which is critical for setting and enforcing regulation — is not yet possible in the U.S., as regulators don’t monitor how the drugs are administered on farms. In fact, much of the information on animal antibiotic use is currently considered confidential business information. Juli Putnam, a spokeswoman with the FDA, said the agency is working with other federal officials to “enhance current data collection efforts,” and “intends to seek public input on such approaches.”

Whether or not the progress in the Netherlands has actually resulted in benefits for public health is not clear. Kluytmans could only state that a previously steady rise in resistant infections in the 2000s had appeared to level off after 2009.

A Harvard paper published in August cautions that more research is needed to fully understand the effect of animal antibiotics on resistant infections. But the review, which was among the documents quoted by the industry group as evidence of animal agriculture’s minimal role in the human health threat, also underscores the limited available data and the need for the U.S. agricultural industry to be more “forthcoming.” It further notes that once antibiotic-resistant strains have emerged, “it might be only a matter of time before they cross the species barrier and adapt to living in humans, at which time there is very little regulation of agriculture can do to prevent their persistence in the clinical setting.”

“The greatest value of reducing agricultural antibiotic use now may be in maintaining a status quo that, while far from ideal,” the researchers write, “is greatly preferable to the alternative.”

In 1970, the U.S. Surgeon General declared that the war against infectious disease had been won. Today, Ebola and antibiotic-resistant superbugs warn us otherwise. While Kluytmans is confident Ebola won’t pose a serious problem in developed nations, antibiotic resistance is another story. He and other infectious disease experts are particularly fearful of emerging resistance to carbapenem, one of today’s last-resort antibiotics.

Spellberg, the Los Angeles doctor who treated the young woman who died from an infection, recalled looking at a printout on the computer screen showing all the antibiotics the implicated bacteria could resist. “Resistant. Resistant. Resistant. Resistant. Resistant. It was resistant to everything,” he said in the new documentary, which is currently screening around the U.S. and will be available to the public in the spring.

“Since penicillin,” he added, “we’ve expected that we’re going to have relatively inexpensive, safe, tremendously effective drugs to treat infections, and this woman had returned to 1935.”