The mystery started the day farmer Russ Kremer got between a jealous boar and a sow in heat.

The boar gored Kremer in the knee with a razor-sharp tusk. The burly pig farmer shrugged it off, figuring: “You pour the blood out of your boot and go on.”

But Kremer’s red-hot leg ballooned to double its size. A strep infection spread, threatening his life and baffling doctors. Two months of multiple antibiotics did virtually nothing.

The answer was flowing in the veins of the boar. The animal had been fed low doses of penicillin, spawning a strain of strep that was resistant to other antibiotics. That drug-resistant germ passed to Kremer.

Like Kremer, more and more Americans – many of them living far from barns and pastures – are at risk from the widespread practice of feeding livestock antibiotics. These animals grow faster, but they can also develop drug-resistant infections that are passed on to people. The issue is now gaining attention because of interest from a new White House administration and a flurry of new research tying antibiotic use in animals to drug resistance in people.

Researchers say the overuse of antibiotics in humans and animals has led to a plague of drug-resistant infections that killed more than 65,000 people in the U.S. last year – more than prostate and breast cancer combined. And in a nation that used about 35 million pounds of antibiotics last year, 70 percent of the drugs – 28 million pounds – went to pigs, chickens and cows. Worldwide, it’s 50 percent.

“This is a living breathing problem, it’s the big bad wolf and it’s knocking at our door,” said Dr. Vance Fowler, an infectious disease specialist at Duke University. “It’s here. It’s arrived.”

The rise in the use of antibiotics is part of a growing problem of soaring drug resistance worldwide, The Associated Press found in a six-month look at the issue. As a result, killer diseases like malaria, tuberculosis and staph are resurging in new and more deadly forms.