The disease started out with the mermaids, although I don't know if it's something their empress engineered or if she just took advantage of it. It forced mermaids who contracted it to feed off of intelligent life like dolphins, humans, and other mermaids - if they didn't, they'd go out of control and start frenzying. Sirens could calm them down, at least up to a point. Mermaids who contracted it were called Stricken.

The Doomwhale was what we called the mostly-undead Stricken-infected whale that the empress used to stimulate the disease and make it spread during that particular Jaunt. At one point, it jumped species and started infecting humans, at which point all hell pretty much broke loose. A group of Travelers did manage to find both a cure and a vaccine, but that only helped the infected on Crownbreaker Isle, which is where the Jaunt took place, and we don't know if this Jaunt is even taking place there, much less whether they still have any vaccine left.