There's no effective way to permanently seal a Primal that's available to current residents of Hydaelyn -- other than, perhaps, executing everyone who worships said Primal. If there were, the Scions or the Garleans would have a solution to the Primal problem. 
Various diseases are referenced in quest lore. It's entirely possible to take one of those and run with it through a variety of means, including Garlean or Allagan modifications (the Garleans intentionally release something; someone stumbles across something the Allagans did); accidental creation via magic of such a thing by arcanima researchers (looking for a cure for another disease, trying to improve Bio); or merely evolution taking its natural course.
I'm always rather wary of server-wide plots because they tend to be for the benefit of their organizers and lean towards powerposing, but that doesn't mean they have to be. It's just that, as Mooncalling said, the plot needs to be planned out in advance with a resolution that anyone can participate in, and one of the rules must be that no one who doesn't want to participate by being infected has to do so.
EDIT: An additional problem with these kinds of plotlines is that I've seen them in past used to push a particular stance on RP. For instance, in CoH, one was used to push the "healing powers don't really heal people, only medicine does" RP stance, which naturally made those who were playing superpowered healers rather unhappy when they could literally do nothing in the plot. Part of the concept needs to be a balance between the eventual solution to the problem and giving both "General Hospital" RPers and "Cure III!" or "Have a potion!" RPers something to do. For instance, you could say, "Healing techniques merely suspend the symptoms of the disease, not its progress; while Esuna, potions, and non-magical healing can make someone feel better for a time, when the disease progresses to its 'coma' phase, the afflicted person drops unconscious. At the coma phase, regular use of healing techniques is required to stave off death." That way, both magical and non-magical healers can contribute equally to the RP.
Another key element, especially if the disease is very dire, is to get buy-in from FCs/LSs. If you can get these groups behind the idea, it'll increase the acceptance of the plot and bring more motivated characters in.
Various diseases are referenced in quest lore. It's entirely possible to take one of those and run with it through a variety of means, including Garlean or Allagan modifications (the Garleans intentionally release something; someone stumbles across something the Allagans did); accidental creation via magic of such a thing by arcanima researchers (looking for a cure for another disease, trying to improve Bio); or merely evolution taking its natural course.
I'm always rather wary of server-wide plots because they tend to be for the benefit of their organizers and lean towards powerposing, but that doesn't mean they have to be. It's just that, as Mooncalling said, the plot needs to be planned out in advance with a resolution that anyone can participate in, and one of the rules must be that no one who doesn't want to participate by being infected has to do so.
EDIT: An additional problem with these kinds of plotlines is that I've seen them in past used to push a particular stance on RP. For instance, in CoH, one was used to push the "healing powers don't really heal people, only medicine does" RP stance, which naturally made those who were playing superpowered healers rather unhappy when they could literally do nothing in the plot. Part of the concept needs to be a balance between the eventual solution to the problem and giving both "General Hospital" RPers and "Cure III!" or "Have a potion!" RPers something to do. For instance, you could say, "Healing techniques merely suspend the symptoms of the disease, not its progress; while Esuna, potions, and non-magical healing can make someone feel better for a time, when the disease progresses to its 'coma' phase, the afflicted person drops unconscious. At the coma phase, regular use of healing techniques is required to stave off death." That way, both magical and non-magical healers can contribute equally to the RP.
Another key element, especially if the disease is very dire, is to get buy-in from FCs/LSs. If you can get these groups behind the idea, it'll increase the acceptance of the plot and bring more motivated characters in.
The Freelance Wizard
Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
