What about Henry's comment about God not being able to see the island?
Based on this comment, and some of the books.....I'm starting to think they're in another dimension.Perhaps a pocket dimension that if it collapses they'll all die. They have to hit the button, otherwise the magnet powers down and the dimension starts to collapse.
They have a limited ability to influence events in our dimension, and the pocket dimension isn't time specific, i.e. the others can influence any point in history and have drawn the survivors together. In some of their initial experimentations they pulled the Black Rock and Rousseau through, etc. They have fined tuned it enough now that they pulled through what they really wanted.....the lost plane.
The cloud monster is an entity from the other dimension.....the others are somehow using the children to communicate with it. The others aren't innocent/good/pure of heart enough for the monster to understand them.
Other entities are communicating with the survivors through the whispers/visions etc, some of it may also be "spill over" from our dimension.....not all of them are "good". The disease is if you listen to the entities too much and essentially lose your mind, what almost happened to Hurley.
Most of the survivors aren't really effected by the disease because there are enough real people together to distract them from the entities' influence. If you're alone you are much more susceptible. Rousseau's party was probably too small to overcome the influence of the entities and they fell victim to the disease.
Yes, probably way off here.....but it's another theory.