Sea Star Wasting Disease (SSWD) is a deadly infection [?] in sea stars (a.k.a. starfishes) and other echinoderms, which leads to mass mortalities.
At least 20 species in total are known to be affected.
To date, it has devastated sea star populations along much of the West coast of North America
The first mass 'plague' was noted in 1972 off the East coast of the US.
The cause is unknown, and no infectious agent has yet been discovered. It's presumed to be either viral, bacterial or fungal. And may have be linked to a genetic mutation ('faulty' gene).
More info US Marine Institute