On the main topic, diseases are one of those "immersive" elements that sounds nice, but doesn't add much in practice. In a game like Skyrim, they are so insignificant you won't know you have one until you scroll through the status screen. More potent diseases, like in Pathfinder, can be annoying early on (ah, ghoul fever when you're too low-level to have restoration spells), but quickly become just another debuff to remove. They don't add any interesting gameplay options or considerations. Try not to get hit, or you might become infected? You were trying not to get hit anyway.
Poisons work better at higher levels imo because these are fast enough to affect characters in combat. Then it becomes a tactical decision: do I spend an action to take an anti-toxin/cast a cure poison spell, or do I keep attacking hoping to kill the enemy before I succumb to the poison?
