A silent killer is spreading throughout the waters of the Caribbean. Known as stony coral tissue loss disease, it afflicts at least 22 species, including some of the largest, oldest, and most important reef-building corals. Infected colonies develop white patches that slowly enlarge, draining the color and life from the animals. In the most susceptible species, such as pillar, brain, and star corals, infected colonies usually die within weeks or months.

It’s the “worst thing I’ve ever seen,” says William Precht, a coral disease specialist in Florida.

Stony coral tissue loss disease, abbreviated as SCTLD, was discovered in the fall of 2014 in corals off Miami. The disease, likely spread by a bacterium or virus or some combination