Major overhaul coming soon, when I get the time next week!
In real life, you get sick. You forget where you placed items. If you go near somewhere contaminated, you have a higher chance of catching something. Some illnesses can even kill. We don't have this in Minecraft. Yet.
With full multiword* support, you can choose a starting world with /plague start <world>*. That world will be marked as Infected. If a world is infected, people standing in water or are in a swamp biome have a small chance of getting infected. Players will also have a chance to get infected when hit*. If players are in a Factions War Zone they have a 5x greater chance of getting infected*
They are unimplemented for now
August 9th
(*) = Subject to change