The Food and Drug Administration is working hard to treat a disease that no longer exists. For the first time, the agency approved a drug to treat smallpox, a disease that was declared globally eradicated from human transmission in 1980. The FDA was blunt in describing why it would go to such lengths just for a precautionary measure, citing the deadly disease’s potential to reemerge in an act of war.

“To address the risk of bioterrorism, Congress has taken steps to enable the development and approval of countermeasures to thwart pathogens that could be employed as weapons,” FDA Commissioner Dr. Scott Gottlieb said in a statement on Friday. “This new treatment affords us an additional option should smallpox ever be used as a bioweapon.”

Smallpox eradication map

The TPOXX drug, whose generic name is tecovirimat, was first tested on animals infected with viruses similar to the variola virus that causes smallpox. Seeing as it is unethical to infect people with an eradicated disease, TPOXX was approved under the FDA’s Animal Rule, which allows animal studies to suffice for an FDA approval when it’s not feasible or ethical to conduct efficacy trials in humans. The drug’s safety was later evaluated in 359 healthy human volunteers without a smallpox infection.

Back in smallpox’s heyday, the highly-contagious disease killed an estimated 30 percent of those who caught it. In the 18th century, an average of 400,000 people died from the disease each year. The disease can spread through the air or even from surfaces that were touched by an infected person, but the US has not suffered an outbreak of smallpox since 1949.

Today, the virus exists in two known laboratories: one at the US Centers for Disease Control and Prevention (CDC) and one in Russia. However, in the wake of multiple deaths in the UK from a Novichok nerve agent attack on a former Russian spy, the international community is fearing the recent uptick in the use of bioweapons. Furthermore, experts have warned that samples of the virus could exist outside of the US and Russia, noting that with access to the right tools, smallpox could be built using modern gene-editing techniques.

“Today’s action reflects the FDA’s commitment to ensuring that the US is prepared for any public health emergency with timely, safe, and effective medical products,” Gottlieb said. The FDA granted this application Fast Track and Priority Review designations to ensure that the drug can be readily available, should it ever be needed.