Whether he's seeing the future, reading minds, or a little of both, Desmond's activation of the failsafe made him "sick," and we're now seeing the symptoms. Locke was sick but got better -- his proximity to the failsafe reinfected him. Eko was already very sick -- I hope he doesn't suffer from overexposure.
And what of Charlie? Shouldn't he be suffering from these symptoms, too? I believe he would -- if he hadn't taken that injection of the "vaccine." That's also what prevented Desmond from getting sick during his original tour of duty in Swan. But I'm guessing he experimented at least once with no injections.
That's when Des painted the Swan mural (eye m sick) which predicted the arrival of Flight 815. Perhaps his vivid dreams disturbed him, prompting him to resume the course of injections (eye m well). Either way, Desmond is now sick with the same disease that afflicted Danielle's crew.
Let us hope our losties respond to his infection with greater understanding than Danielle...