The 'disease' causing havoc wasn't a disease at all. It was just Fester's [Festering Aura] that had turned all food into literal poison, with a hint of weakened undead virus sprinkled on top for garnish.
Since the virus was weakened, it explained the slow spread of the disease and how the dead didn't turn into the undead as they should have. The [Weakened Virus] was one of the many things Blaze had experimented with during his travel to the Sadarla.
He had already used it once on the tribes that bullied Deca and knew they'd be effective. But there was one issue.
At first, Blaze had been planning to just release the virus and let it do its work. But unlike the unprepared tribes, the Solia kingdom had strangely gone undiseased for centuries.
He believed they must have methods to cleanse most diseases and didn't want to take any chances. Hence, he sent Fester there.
Even if they somehow cured the disease, Fester's presence alone would kill them all.
