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ROBERT SIEGEL, host: Human beings occasionally get diseases from animals - think of swine flu, rabies, anthrax. And while it's rare, it turns out humans can also spread disease to wildlife, with grim results. NPR's Richard Harris reports on a study about a disease that is killing coral.

RICHARD HARRIS: Coral reefs throughout the Caribbean have been suffering sharp declines in recent years and one of the most devastating causes is a disease called white pox.

KATHRYN SUTHERLAND: White pox disease was first described in the Florida Keys in 1996.

HARRIS: Kathryn Sutherland is a coral reef ecologist at Rollins College in Florida.

SUTHERLAND: And since that time, elkhorn coral, the species that it affects, has declined on average of 88 percent in the Florida Keys and we are seeing similar declines elsewhere in the Caribbean.

HARRIS: The coral is named for its resemblance to elk antlers and is among the most important reef-building species.

Sutherland and her colleagues soon found a culprit for the die-off, a bacterium called Serratia marcescens. It also happens to cause disease in human beings, notably hospital infections. But the scientists couldn't prove cause and effect.

SUTHERLAND: In 2002, we could only speculate that human waste was the source of the pathogen because the pathogen is also found in the guts of other animals.

HARRIS: Like deer on the Florida Keys, so Sutherland and some colleagues exposed the coral in the laboratory to bacteria extracted from sewage and they report in the journal PLoS ONE, the coral got the pox within days.

SUTHERLAND: So that gave us definitive evidence that white pox disease is caused by a pathogen found in human sewage.

JAMES PORTER: This is almost the man-bites-dog story. This is exactly the opposite from the way it usually is.

HARRIS: James Porter of the University of Georgia also worked on the research and he says the bacteria overcame all sorts of hurdles to make this leap.

PORTER: This is a very rare and unusual evolutionary triple jump. It went from humans to the lower invertebrates, coral. It went from the terrestrial environment where we live to the marine environment and then it went from the anaerobic conditions of our stomach to the fully oxygenated conditions on the reef.

HARRIS: Porter says they are still trying to explain exactly how the bacteria makes coral sick and why white pox appeared so suddenly and viciously. The good news is you don't need to answer those questions to protect coral. You can just keep the germs away and, happily, that's been happening along the Florida Keys.

Jay Gewin, Utilities Manager in the city of Key West, says residents there voted to upgrade their leaky sewer system, not to save coral, per se, but to get rid of health warnings that were keeping the tourists away.

JAY GEWIN: In the early 2000s when this was such a problem, every single beach in the city of Key West would have an advisory.

HARRIS: Those health warnings on the beach are now rare, he says, thanks to a sophisticated new waste water system that cost the city more than $70 million.

GEWIN: The waste water is treated to the highest level in the state of Florida and then it goes into a deep injection well where the treated final water product is sent down thousands of feet into the ground.

HARRIS: Eventually, it may seep back into the ocean, but he says, by then, it's clean. The biologists say that since the new system was installed, elkhorn corals have stopped further die-offs around Key West. And Kathryn Sutherland says other towns along the Keys are now in the process of cleaning up their act, too, which is a helpful step.

SUTHERLAND: But this is a problem Caribbean-wide and there's a widespread lack of waste water treatment in the wider Caribbean region.

HARRIS: And that's bad news for the elkhorn coral. Due largely to the disease spreading from humans, it's been tagged as vulnerable on the Endangered Species List.

Richard Harris, NPR News.

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