A dangerous, unidentified disease is ravaging coral reefs in the Florida Keys, the third-largest barrier reef system in the world. The disease, which appears to be bacterial, threatens to wipe out significant portions of the state’s coral populations. Scientists still don’t know much about how it spreads — or what it even is — but what they do know is that it looks like about half of Florida’s coral species are susceptible to it.

The strange infection was first reported in 2014, when it began spreading like wildfire. In November 2017, scientists thought its run had come to an end when it reached a gap in the 360-mile tract of barrier reef near the eastern end of the Seven Mile Bridge. But in April, the Miami Herald reported that the disease had jumped the gap, wreaking gnarly effects on the corals on the western edge. “When they’re affected by this, the tissue sloughs off the skeleton,” Erinn Muller, Ph.D., the science director of the Mote Marine Lab’s Elizabeth Moore International Center for Coral Reef Research and Restoration, told NPR on Tuesday.

This image of elliptical star coral from Marathon, east of the Seven Mile Bridge, shows just how quickly all the living tissue sloughs off infected corals.

Corals are living animals — technically, thousands of tiny, identical polyps all living on the same skeleton. That skeleton is created by members of the colony that secrete calcium carbonate, which eventually builds up over time, creating the animal’s “bone.” The polyps in colonies, generally being genetically identical, tend to respond in the same way to dangerous new infections. As Muller explained to NPR, that’s exactly what’s happening with this new disease: “And we see that once a coral is infected, it usually kills the entire coral, sometimes within weeks. And it