Sick. Sick. Sick.

You make a good point. Being sick isn't modeled accurately enough in most games. Debuffs tend to be stat focused and fail to immerse the player in the misery that is a full on head cold. When I try to think of an interesting use of disease lately all I can think of is the Zul'Gurub plague... and that wasn't really interesting, just annoying. "Curse of The Blackheart" was one of the most unique debuffs I saw in WoW, every once in a while after receiving it you would get "chest pains" and would crouch over stunned.

I've always thought that the cold you could catch in the Metal Gear games fit the gameplay very well. If you stood around in the cold and wet for a while you would eventually get a cold and start sneezing. While that would just be pointless in a game like WoW, in MGS it was pure comic gold as that sneeze caused a clearing team to rain death upon your location with bullets.

Now that I really think about it, all of the Forsaken in WoW came from a disease. An undead plague seems like it would be more interesting if it were a dynamic force though. Opposing factions could drop of a crate of diseased rice and the next thing you know the whole town has become zombies and you have to cull it before the disease spreads further. Players could be indirectly affected if they buy from NPC food or drink vendors during the infectious period.

Besides infecting resources like food and water you could also infect traveling merchants and other players who might not even know. The right setup before a confrontation could find your opponents unable to speak due to a strange onset of lockjaw. If certain classes can detect disease better then others a player might find themselves unknowingly shot to death outside of their own base in order to prevent the guild from being devastated by a biological attack. Blankets with smallpox and all that good stuff...

When I think of extremes now my motto is WWDFD? (What Would Dwarf Fortress Do?), so there might as well be some kind of ability to give your enemies diarrhea in the heat of battle. While clutching your soiled chain mail in vain you get cleaved in half by another player. Should the ability to fight while crapping your pants be a fortitude check or some kind of acquired talent?

The detection aspect leads to it own psychological side effects. Unlike seeing invisible units, disease detection could be based off of ones skill in that area. A higher level player would be able to detect diseases that a low level could not and because of this could also lie about diseases that don't actually exist. A "The Thing" or a Scarlet Crusade sort of scenario could be created where players are willing to murder their comrades at the word of their leader for fear of infection from outside sources. If you created diseases that caused players to become powerful uncontrollable zombies then all the more reason to put them down before they turn.

Abilities that kill players are easy... the ones that make players kill each other are priceless.

Would it make you feel better if NetDevil made a Lego MMO?