Coral reef ecologists have found a potential answer to a persistent and troubling puzzle. The elkhorn coral, named for its resemblance to elk antlers and known for providing valuable marine habitat, was once the Caribbean’s most abundant reef builder. But it has declined 90% over the past decade, in part due to highly contagious white pox disease, which causes large lesions that bare the coral’s white skeleton and kill its tissue. Now, after nearly a decade of data collection and analysis, researchers have a possible cause of the affliction: human excrement. The finding represents the first example of human-to-invertebrate disease transmission and suggests a practical approach for halting the disease’s spread.