Now, lets bring this into WoW. Realistically, as in the Middle Ages, Capital Cities aren't as sanitary as they are in-game. You have to take into consideration of the horse fecies on the streets, sewage, waste and other disgusting things. Not to say bathing isn't done on a regular basis, I mean come-on. How often do you ever go for a swim ICly or bathe? Be honest.
Now, the Plague started with infested Flower/Rice shipped to Lordaeron from Northrend under disguise of a regular, possibly Kul'Tirian Merchant-Ship. Now, animals, I.E - Rats / Rabbits, etc, often live on these ships. Tis' how the Bubonic Plague hit Europe from Asia in the Middle Ages. They often fed on the left-overs of infected goods, then when docked, they scurry off into the night, and back to sanitary conditions of Cities : Fleas are almost evident. Fleas bite these Rats and bring them around all over Azeroth realistically, so it is impossible for someone to say," You are safe from the Plague/Scourge here. "
Now, I am not saying I am all to with some Necromancer brewing some potent Plague and all, I am simply posting on some facts of the hazards of Capital Cities.
I, personally, would like to see some people RP sick, or see some sort of Guild devoted to organization within cities. ( 'Cept for Kal'dorei. Due to their common nature, their city won't be as bad as Stormwind for say, compairing Stormwind ( present-day ) to Mid 1300's to late 1450s London.
The soul purpose of this post is just to inform people that no-matter where you are, or what you do, there will always be danger lurking. As for the Forsaken RPers. Plague isn't healthy for the dead either. It can be like Saronite to mortal, in a way. Drive you mad from ever-suffering agony, because it is confirmed through IC cenimatics that Forsaken/Undead things -feel- pain. If not, it'll further rot your body until you are a pile of Horde-Sludge in the sewers of Undercity... Whew.. So that's what that green stuff is, I bet!
A link of basic information and symptems of the Plague. Yes, it does cause rotting to increase, and occur. : http://en.wikipedia.org/wiki/Bubonic_plague