UNTREATABLE NO MORE. Fear of another smallpox pandemic keep you up at night? You’ll be happy to hear the FDA approved tecovirimat (Tpoxx) on Friday. It’s the first drug designed to treat the infectious disease, which kills about 30 percent of the people who contract it.

Though not tested directly against smallpox in humans, in trials, the smallpox treatment did increase the survival rate of primates and rabbits injected with two similar diseases (monkeypox and rabbitpox, respectively). And the drug produced only minor side effects when evaluated on 359 healthy human volunteers.

A DEADLY WEAPON. Before this drug was approved, experts considered smallpox untreatable and incurable. Our only defense against the disease was vaccination, but thankfully, that proved to be a powerful weapon — in 1980, the World Health Organization (WHO) was able to declare smallpox fully eradicated.

So, why did the FDA just approve a smallpox treatment if no one in the world has smallpox?

Smallpox may no longer exist in the wild, but the virus does still exist in the lab. Right now, we know of at least two labs with sample of smallpox in their freezers, one in Russia and one at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia. Other samples could exist elsewhere, and there’s also the possibility that someone could use the latest gene-editing technology to recreate the virus.

Because most smallpox vaccination efforts ended when the disease was officially eradicated in 1980, most people born after that date are not protected against it. That means smallpox has the potential to cause a pandemic if the virus ever makes its way back into the general population. While this escape could be accidental, there’s also a chance it could be a deliberate act of bioterrorism.

BETTER SAFE THAN SORRY. So, if smallpox is still such a threat, why don’t we just keep vaccinating everyone?

Well, the smallpox vaccine is dangerous, for one. It has the potential to cause serious, even deadly, side effects. And not everyone can receive the vaccine. As Peter J. Hotez, dean of Baylor College of Medicine’s National School of Tropical Medicine, told the New York Times, we can’t give the smallpox vaccine to people who are pregnant, H.I.V. positive, undergoing cancer treatments, or with other health conditions.

Hopefully no one ever needs to use Tpoxx. But knowing a smallpox treatment exists should help us all sleep a bit better at night.

READ MORE: Drug to Treat Smallpox Approved by F.D.A., a Move Against Bioterrorism [The New York Times]

More on biological weapons: Synthetic Biological Weapons May Be Coming. Here’s How To Fight Them.