I think that the island has powers capable of healing physical ailments. I think the "sickness" is mental.

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"Not all who wander are lost." - J. R. R. Tolkien

"You cannot live a perfect day without doing something for someone who will never be able to repay you." - John Wooden

Okay please, forgive me as I try to express an odd and fleeting thought I just had. You know how some people hate the sound of bagpipes or better yet, nails on a chalk board? It's never bothered me but, I have seen people cry out in real pain.
What if the magnetic feild is kinda like that. It heals some and drives others mad. It may put someone on edge and affect them just enough to pop a screw loose. It's just a thought.

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Hope is a very dangerous thing to lose. - Sayid

I do what my Rice Krispies tell me to. - Anonymous

That might explain why Henry is trying to take out Locke mentally. He is obviously trying to muddle him badly, as seen by his sly grin. Maybe Locke (who was healed and seems to be the smartest) needed an outside force to "pop a screw loose".