Testing anthrax drugs in humans isn’t just hard. It’s impossible. You can’t give people anthrax because, well, duh, ethics. But you can’t test it on people who already have anthrax, either, because it’s rare—unless you have some kind of bioterrorism attack, in which case, too late!

So you want to test, in people, a drug for a rare but deadly disease. What do you do?

Ask the Food and Drug Administration, and they’ll tell you: You don’t. After the anthrax attacks that followed 9/11, the agency established its “Animal Rule:” If the disease is too rare and deadly to test a drug in humans, drugmakers can prove it works using just animal data, as along they can also show the drug is not dangerous to humans.

Now, more than a decade later, the FDA has approved a vaccine under the Animal Rule. It’s an anthrax vaccine called BioThrax, and used with antibiotics it’s supposed to keep people from getting sick after they get exposed to Bacillus anthracis, the bacteria that causes the disease. Antibiotics beat back anthrax immediately, but the bacteria famously goes dormant, hiding inside spores for weeks or even months. The vaccine confers long-term protection.

BioThrax isn’t new. The vaccine has been around since 1970 as a preemptive measure against anthrax before exposure for people at high risk: anthrax researchers, vets, anyone who handles livestock. Since 2006, after an Institute of Medicine review put to rest doubts about the vaccine’s safety and efficacy, BioThrax has been mandatory for everyone in the military.

So why is a decades-old anthrax vaccine getting approved for a new use now—more than a decade after the nation’s last anthrax scare? “That’s indicative of how long it takes to approve vaccines and drugs,” says Gigi Gronvall, a biosafety expert at the UPMC Center for Health Security. Despite allowing drugmakers to skip human trials, the Animal Rule is apparently no shortcut.

That’s because, for one thing, animal trials under the Animal Rule actually look a lot like human trials. As the FDA was implementing the Rule, one point of contention was whether researchers would need “basically an ICU for animals,” says Gronvall. The logic goes that humans sick with anthrax would go to the hospital, so if you’re using survival rates among vaccinated monkeys as an analog for survival rates among vaccinated humans, the monkeys should get the same kind of care. The FDA ultimately decided that the medical care doesn’t have to be as intensive as it would be for humans, but it goes as far as suggesting blood transfusions for the research animals as necessary.

Researchers also have to prove they’re using the right animal models in the first place. That means anthrax has the “same symptoms, same pathogenesis, same progression of disease” in the test animal as in humans, says James Roth, director of the Center for Food Security and Public Health at Iowa State University. Mice, a very common lab animal, are out—anthrax spores don’t give them the same symptoms. Drugmakers also need data from at least two animal models to cover all their bases. For BioThrax, that was monkeys and rabbits.

The FDA has approved a nine other treatments through the Animal Rule—including drugs for the plague, cyanide poisoning and radiation exposure—but BioThrax is the first vaccine. Now that the FDA has mapped out the path, other drugmakers can more easily follow. “People have talked about using it for West Nile and other diseases that don’t happen predictably enough,” says Gronvall. In fact, although FDA created the Animal Rule after the anthrax scare, it’s not exclusive to biodefense. If someone is looking for something rare and deadly to take a crack at, there’s always Ebola.