Several meters underwater off the coast of Bonaire, a small island in the south Caribbean, Danielle de Kool floated in place in front of a large head of boulder brain coral. The pattern across its surface looked like the maze you might find on the back of a cereal box.

From a plastic syringe, de Kool, an ecologist at a local environmental group, squeezed a toothpaste-like substance into her hand. She then pressed the paste onto the surface of the coral around the edge of a large white splotch that had recently appeared.

The coral was sick. And this paste might help heal it.

In the last decade, a mysterious illness called stony coral tissue loss disease (SCTLD) has been ravaging coral in reefs across the Caribbean. The disease — which is likely caused by a bacterium or virus — targets a number of hard, reef-forming coral species. It essentially pulverizes the soft coral tissue, killing centuries-old colonies in a matter of weeks. The plight has now spread to at least 30 countries and territories in the Caribbean, where corals were already suffering from pollution, climate change, and other threats. In regions hit by SCTLD, the disease has reduced the area of coral by anywhere from 30 percent to 60 percent.

Last spring, the disease was spotted in Bonaire — one of the few spots in the Caribbean where you can still find an abundance of healthy coral. The island, like many others in the tropics, is deeply dependent on its reef. Tourism is the engine of Bonaire’s economy, and the majority of visitors come to scuba dive and snorkel. Plus, large coral structures dampen waves that hit the shore, lessening flooding during big storms.

SCTLD has already killed off more than 90 percent of some coral species in Bonaire, including boulder brain and maze corals, according to preliminary data from STINAPA, a local organization working to protect the reef. The outbreak is quite literally threatening Bonaire’s way of life, the primary source of income for its residents, and the island’s ability to defend itself from destructive hurricanes.

With the stakes so high, ecologists across the Caribbean are trying desperately to ease the spread of SCTLD. And on that morning in July, de Kool, who works for STINAPA, was doing one of the few things that seems to work: smearing sick corals with antibiotics.

Each bit of coral is a colony of animals, comprising hundreds to thousands of tiny creatures called polyps. Those polyps produce skeletons made of calcium carbonate — the same material found in sea shells — which forms the hard structure of the reef.

And like other animals, corals can get sick. Over the last century, a number of diseases have decimated coral populations worldwide. White band disease, for example, first appeared in the 1970s and has since killed more than 80 percent of staghorn and elkhorn corals in the Caribbean. These iconic species, named for their antler-like appearance, were once so abundant in the shallows that fishermen would have to cut them down in order to clear a path for their boats.

SCTLD, meanwhile, is relatively new. Scientists first observed the disease a decade ago in Florida, and there are still many unknowns, such as where it first came from and even what SCTLD is. It could be a bacterium or a virus, or both working together. Some kind of bacteria, for example, could be making coral more susceptible to a virus, said Blake Ushijima, a microbiologist at the University of North Carolina Wilmington.

Scientists also aren’t sure how SCTLD has moved around the Caribbean. The spread generally seems to follow ocean currents, but it sometimes jumps between distant places, said Marilyn Brandt, a coral scientist at the University of the Virgin Islands. In some cases, cargo ships are likely responsible for the spread, she said. As ships load and unload cargo, they fill and empty ballast tanks that help stabilize the vessels. These tanks could be inadvertently transporting SCTLD-causing pathogens. (There are now regulations and technologies designed to minimize the spread of disease and invasive species in ballast water, though not all ships adhere to them, Brandt said.)

On that morning in July, I was diving with de Kool on a reef just off the northwest coast of Bonaire. The view underwater was stunning: a messy tapestry of colorful corals and sea sponges home to all kinds of sea creatures. About 20 minutes into our dive, a hammerhead shark swam by.

Yet there were also signs of SCTLD everywhere.

Reefs get their signature bright coloring from a symbiotic algae that lives inside the live coral tissue. The sickened colonies, though, had big white spots where the disease had apparently destroyed the tissue, exposing the coral’s bone-white skeleton. Many of those spots were already turning green from different kinds of algae that grow on dead sections of coral.

“I’ve never seen a disease like this,” Caren Eckrich, an ecologist at STINAPA, told me. In Bonaire, it’s something close to an “extinction-level event,” she said, meaning it’s nearly wiping out some of the island’s coral species.

But scientists are not totally powerless against it. They have a weapon.

For years now, companies and hobbyists who grow coral in aquariums have used various antibiotics to treat sick sea creatures, including coral, Ushijima told me. They essentially dip pieces of coral into antibiotic washes or put medicine directly into the fish tanks.

When SCTLD began spreading several years ago, scientists tried a similar approach — and it worked. They brought sick corals infected with SCTLD into the lab and treated them with antibiotics, including amoxicillin, the same drug humans use for bacterial infections. Most of them recovered.

Treating corals in the wild, however, is a different challenge altogether. That’s where that toothpaste-like substance de Kool was using comes in. Through trial and error, scientists figured out that they could mix powdered amoxicillin with a biodegradable putty, made by the company Ocean Alchemists, that sticks to the surface of coral underwater. When you apply the antibiotic paste around a SCTLD lesion, it can, as studies have shown, stop or slow the disease from spreading through the colony. “It is very effective,” Brandt said.

That amoxicillin works is actually a bit peculiar. While it’s still not clear what pathogen causes SCTLD, there’s some evidence suggesting the disease is viral, Brandt said. How would amoxicillin, which kills bacteria, stop a viral disease? One theory, she said, is that if it is indeed viral, the pathogen may still require bacteria to cause disease (in humans, bacteria and viruses sometimes cooperate with each other). Another possible explanation, she said, is that the pathogen targets the symbiotic algae living within coral tissue. Antibiotics often kill those algae without killing the coral, essentially removing the target of infection and stemming the spread. Or perhaps SCTLD is caused by bacteria after all, as other scientists suspect. No one knows for sure.

“It seems very likely that the bacterial component is at least very important in the infection process,” said Karen Neely, a research scientist at Florida’s Nova Southeastern University who first trialed the antibiotic paste in the wild. “But regardless, the amoxicillin does work. It’s keeping corals alive.”

Back on the reef, I watched de Kool and a handful of other divers, including dive instructor and educator Carmen Toanchina, cruise around the reef and treat corals. They’d spot a colony with white lesions, unclip a syringe from their vest, squirt out some paste, and then, somewhat awkwardly, try to apply it to the coral’s surface. It was like watching someone stick strips of Play-Doh on weird-looking rocks but underwater — where masks fog up, sharks swim by, and one deep breath threatens your buoyancy. The work was slow going.

These treatments appear to be working, said Jeannine Toy, who oversees a squadron of STINAPA volunteers like Toanchina who apply the antibiotics. They’ve been treating reefs in Bonaire for more than a year now. “About 70 percent are healed after we treat them,” Toy told me after the dive.

The goal isn’t to treat every coral around the island, Toy said — that’d be nearly impossible. Rather, STINAPA wants to treat enough colonies so that there are plenty of live corals to spawn, or sexually reproduce, and create the next generation of corals in Bonaire.

The bad news is that SCTLD is unlikely to disappear anytime soon. It’s now endemic, or consistently present, in some regions, like Florida and the US Virgin Islands. Scientists also fear that it will soon spread to the Pacific, home to the Great Barrier Reef and, in general, a much higher diversity of corals. It’s not clear how susceptible Pacific corals will be. “The scary part is that we don’t know,” Ushijima said.

Against the enormity of the ongoing outbreak, antibiotics are sorely inadequate. While amoxicillin can stem the growth of lesions, it doesn’t prevent infection. And applying the paste is incredibly labor intensive, Brandt said. “It requires tons of divers,” she said. “I have four people and that’s all they do.”

Some scientists, including Ushijima, are also concerned that the bacteria it kills might eventually develop a resistance to amoxicillin, making the treatment less useful. (So far there are no signs of antibiotic resistance, Neely says.)

For now, Brandt points out, giving sick corals antibiotics is the best option available. “It was the only effective solution that we were able to deploy in a large way,” she said, referring to her conservation work in the US Virgin Islands. Her team, she said, “has saved quite a lot of coral.”

Meanwhile, scientists like Ushijima are also working on other potential treatments, such as coral probiotics. Some corals appear to be naturally resistant to SCTLD; the microbes found in and around them may have something to do with this resistance. Certain kinds of bacteria, for example, help corals fight off disease, Ushijima said. Biologists are trying to identify those defense microbes so they can inoculate wild coral with them.

This approach points to something hopeful: Some corals are doing just fine.

Again, this could have to do with those microbes or with genetics; resistance to disease can be rooted in coral DNA. But it also has to do with the environment, Brandt said, and the other threats corals are exposed to. Reefs that are already weakened by extreme heat or pollution are more likely to get sick, just as it’s easier to catch a cold when you’re stressed. What’s more, Brandt said, is that reducing local sources of stress gives corals a better chance of growing back after they suffer a loss from SCTLD.

If any reef can survive the impacts of SCTLD, it’s Bonaire. The island has protected its reef from threats like overfishing for more than half a century, longer than pretty much any other region worldwide. And the corals here have demonstrated that they can bounce back from major die-offs, as I recently reported.

As de Kool and I cruised around the reef, she wasn’t only treating sick corals but also monitoring colonies that have so far resisted infection. There were a lot of them, including big heads of brain coral and even some pillar corals, which have been hit especially hard by SCTLD in the Caribbean. Perhaps these colonies are resistant to the disease. Perhaps they will seed the next generation of corals around the island, helping this once-vibrant reef recover.

If not, doctors are standing by with medicine.

“I spent a good part of my career monitoring corals to death,” Neely told me. “We can’t do that anymore. We have to be active. We are part of the reason that reefs are dying and to not do something about it is really just unacceptable at this point.”