After nearly 16 years, the U.S. has agreed to import beef from Ireland—the first European country to get the go-ahead since the epidemic of mad cow disease swept the continent In the 1980s and 1990s. The move—which may extend to the rest of the British Isles later this year—serves as a milestone in the history of the disease, which can destroy not only the brains of cows, but also those of the humans who eat them.

The epidemic exploded because of the way British farmers raised their bovines. Instead of eating grass or grain, the animals consumed feed made from their dead brethren. Coupled with money-saving changes that did not fully cleanse the feed during processing, the forced cannibalism spread the pathological agent, a protein called a prion. Arguably the sturdiest pathogen on the planet, prions can survive standard sterilization in the lab and endure in the open for years. Once in the body, prions can destroy the brain slowly, over a decade or more, causing early dementia and ultimately death.

To control the mad-cow outbreak—which reached global proportions because the U.K. exported its prion-tainted feed to several countries—governments outlawed the practice of cow cannibalism, slaughtered millions of potentially infected bovines and changed slaughterhouse practices so that tissue where prions congregate, such as the brain and spinal cord, would be separated out and discarded. In the past, those parts often ended up in the human food supply in ground meat and sausages.

Those measures worked, if slowly. According to the latest statistics from the World Organization for Animal Health, the number of cases of bovine spongiform encephalopathy (BSE), as the disease is formally known, declined in the U.K. from more than 37,000 in 1992 to just 11 in 2010. From 2010 to 2013, the world saw only 100 mad cows.

Nine of those 100 cows, however, came from Ireland, which the U.S. cleared for beef imports earlier this week. What's more, Bloomberg News reports that the U.S. will probably check British plants later this year in preparation for allowing beef imports from the U.K, where it all began. (For comparison's sake, the U.S. had one BSE case in that period.)

So you might wonder why the U.S. should reopen the beef trade with these countries. It's not as if the U.S. needs more steaks; the nation produces nearly 26 billion pounds (about 12 billion kilograms) and exports 10 percent of that each year. Imports, mostly from Canada, amount to a small 2.2 billion pounds, according to the U.S. Department of Agriculture.

Rather, the reason is money. As the Irish food ministry puts it, the U.S. is "a huge prize given the size of the market and the demand we know exists there for premium grass-fed beef." The U.S. decision allows Irish authorities to approve beef plants that presumably will adhere to practices that minimize the BSE risk. Grass feeding would be one of those.

But perhaps the best sign that we can live with the risk is that, despite the exposure of tens of millions of burger-eating Brits to prions during the height of the mad cow epidemic two decades ago, only 229 people worldwide have contracted the human form of the ailment (called variant Creutzfeldt-Jakob disease). A feared second wave of human cases based on a genetic profile of susceptibility has not materialized. As time goes by, the odds that people will develop the brain ailment from having eaten tainted beef diminishes. Exactly why only some people got sick remains a mystery.

Does that mean the U.S. should throw its doors open to beef imports? We're fortunate that, when eaten, mad cow prions do not readily infect people; there were plenty of legitimate scientific concerns 20 years ago about the possibility of a time bomb of human deaths. Feeding practices instituted for financial reasons set off a series of unanticipated changes that triggered the mad cow epidemic, catching the world off-guard. In the latest move to benefit the economics of global trade, regulators will need to be vigilant to avoid unleashing any new horrors.