But to Lamb’s surprise, no one had really estimated the extent of plastic junk on coral reefs, or studied how that junk relates to disease. Indeed, when people think about the many ills befalling reefs—rising temperatures, acidifying waters, overfishing, nutrient pollution, ravenous starfish—plastic rarely gets a mention.

Worse still, plastic-induced ailments disproportionately affect the corals that provide important habitats for fish—the ones that create intricate branches and layers. That same architectural complexity becomes easily filled and entwined with plastic, so these corals are eight times more likely to be diseased than simpler ones with rounded shapes.

In fairness, not all reefs are strewn with plastic. Lamb never saw much of it in the Great Barrier Reef, for example. But “in Indonesia, we were seeing 26 pieces of plastic in an area the size of a Manhattan two-bedroom apartment,” she says. The Asia-Pacific region is home to eight of the world’s top 10 plastic-polluting countries, and more than half of the world’s coral reefs. According to estimates from Lamb’s team, those reefs are currently covered with around 11 billion pieces of plastic—and that’s likely a gross underestimate because it doesn’t account for contributions from China, the world’s top plastic polluter.

This problem is only going to get worse. Half of all the plastic that has ever existed was made in just the last 13 years, and production is still accelerating. Around nine tons are created every second. By 2025, Lamb’s team estimated that around 15.7 billion pieces will be entangled in Asia-Pacific reefs.

It’s not clear how that plastic actually leads to disease, and the exact route probably differs from one infection to the other. In black-band disease, a mat of dark algae moves across a coral, leaving dead white tissue in its wake. Those algae thrive in dark, low-oxygen environments—the kind you’d find under a piece of plastic. Skeletal eroding band disease also involves a spreading wave of death with a white trail, but it is caused by a protozoan that invades through wounded tissue. Perhaps sharp plastic fragments are cutting the corals, giving the protozoan an easy entrance.

Other diseases, like the aptly named white syndrome, are caused by opportunistic bacteria like Vibrio, which have a penchant for hitchhiking on plastic debris. “Plastic could be perfect rafts, full of pores and pits that are beneficial to the Vibrio, which they then introduce into the corals,” says Lamb.

“This is a very important study for coral-reef biologists and managers, as it evaluates a driver of disease that has never been addressed,” says Rebecca Vega Thurber, from Oregon State University, who studies the microbes that live on corals.