The Food and Drug Administration has approved the first-ever drug to treat smallpox, a disease that no longer exists.

It’s a just-in-case decision, made to ensure that there is a drug to treat people if smallpox were ever used by bioterrorists, the FDA said.

“This new treatment affords us an additional option should smallpox ever be used as a bioweapon,” FDA Commissioner Dr. Scott Gottlieb said in a statement.

The drug is called Tpoxx. Its generic name is tecovirimat. Because smallpox no longer infects people, it was tested for safety in 359 volunteers, and tested for efficacy in animals infected with close cousins of smallpox.

For drugs to treat diseases such as smallpox, which cannot ethically be tested in people by infecting them on purpose, the FDA has special rules. If the drug can be shown to be safe in people, and if it works to treat disease in animals, the drug’s maker can seek approval.

#FDA approves the first drug with an indication for treatment of smallpox: https://t.co/6Cwd8UuyyQ. pic.twitter.com/ZjM5IIA51r — FDA Drug Information (@FDA_Drug_Info) July 13, 2018

Smallpox was eradicated in 1980, after a sustained global vaccination campaign. The last known naturally occurring case was in 1977. The last outbreak in the U.S. was in 1949.

These days, most people are not even vaccinated against smallpox. But samples are still in storage, and bioterrorism experts say some governments, including the former Soviet Union, have in the past worked to develop the virus into a biological weapon. And because samples are in storage, there is always the possibility of an accident.

When it was still circulating, smallpox was a terrible disease. It killed an estimated 30 percent of people who caught it, and scarred many of the survivors.

Smallpox killed as many as 500 million people in the 20th century alone.

It causes fluid-filled pustules that can cover the whole body. The virus is highly contagious, spreading in the air, through direct contact and on surfaces that have been touched by an infected person.

The first real vaccine was one against smallpox, made by Edward Jenner in 1796 from a related virus, cowpox. Current vaccines are made from another related virus called vaccinia.

There has never before been a specific treatment for smallpox. In the 20th century, doctors provided "supportive care" for patients, keeping them hydrated and lowering fevers.

Tpoxx is a pill developed by SIGA Technologies with funding from the federal government’s Biomedical Advanced Research and Development Authority.

“Additionally, under Project Bioshield, BARDA has acquired two million courses of oral Tpoxx and such courses have been delivered to the strategic national stockpile,” the company said in a statement.