basically, there's a "madness god" that exists in the ever present glow in the world.
Those in the glow too long can succumb to the glowmadness. According to the Savage Worlds rules, there are four steps; irritable, angered, enraged, and finally glowmad.
Each step causes the creature to have less and less control over his anger until the madness takes him, transforming him into a mindless, freakish brute bent on destruction.
Specifically, I don't want this to be a very "deadly" disease, because it'd be a short campaign. I'd like it to be something that affects the players rarely, but prolonged exposure can increase the dangers.
Its an interesting concept.
A psychological condition brought on by exposure to a specific trigger environment or substance.
Trigger: Contact with Glowmadness substance, whether that be walking in luminessant long grasses, breathing tiny funal spores, standing in the spray from a strange waterfall, examining a strange curling symbol painted on teh underside of a pepper pot etc.
An attack vs. Will of an appropriate DC would handle whether you contract Glowmadness or not.
You have already stated your stages:
(-2) Cured - no longer suferring from the effect
(-1) Recovering - well along the path towards making a full recovery
(initial) Irritable - initial state of the condition
(+1) Angered
(+2) Enraged
(+3) Glowmad
You just need to work out appropriate effects for each stage or at least Irritable, Angered, ENraged and Glowmad.
For treating the disease I would use
Heal for someone else treating you or
Insight if you are treating yourself. I would use a DC based off either the level of the effect or maybe just have dependant on the level of the PC?
Possible Effects:
Irritable
You are frequently on edge and easily annoyed. Take a -1 penalty to Will defence and all social skill checks.
Angered
You seem to be in a permanent bad mood. Take a -1 penalty to Will defence and a -2 penalty to all social skill checks, if you ever fail a repeatable skill check you take a cumulative additional -1 penalty to further attempts.
Enraged
Your attitude makes talking to you unpleasent even for your friends. Take a -2 penalty to Will defence and a -4 penalty to social skill checks, if you ever fail a skill check you become too angry to attempt the same thing again and usually resort to some sort of violent outburst.
While you are bloodied you become Enraged. While Enraged you gain a +2 bonus to attack rolls and a +4 bonus to damage against any creature that attacked you last round, you instead take a -4 penalty to attack rolls and no bonus damage against targets that didn't attack you last round.
(sorry that could probably be worded better)
Glowmad
You take on an almost bestial expression and people naturally give you a very wide birth. Take a -4 penalty to Will defence and a -8 penalty to social skill checks, if you ever fail a skill check you become too angry to attempt the same thing again and usually resort to some sort of extreme violent outburst.
While you are bloodied you become Glowmad. While Glowmad you must attack the nearest creature (friend or foe) each round if 2 or more creatures are closest determine randomly, you gain a +4 bonus to attack rolls and deal +8 damage
Other considerations:
You could add the rules that if someone is already suffering Glowmadness and they come into contact with another trigger (once per encounter) then they automatically progress to the next stage on the condition track immediately.
You could also add a rare substance that can counteract Glowmadness, you could call it Glowcalm. One application automatically reduces the effect of Glowmadness and immediately moves the condition one step towards being cured.
Anyway this is teh starting point I would take for your idea. I hope it helps.