I have heard all sort of speculation that Sayid is dying from this "illness" that he has aquired...my thought...MIB and Jacob are getting there teams or "armys" together. We have followed Jacob for most of the time....his people the others and the newly found people at the Temple. The side we are just learning about is MIB. Rousseau was apart of that i believe..Just like Sayid and Claire are now. I have heard talk of tehm slowly dying and not being who they are anymore. I think they are still who they are they just have different priorities...working for MIB, so we know three people who are on his "team" claire, sayid and the late Locke who is MIB. I think he will get Sawyer and Jin to even the score.
How they are choosen? Not sure, maybe the Spring, the water was tainted and therefore Sayid recieved his illness, perhaps claire and rousseau drank some of the water also?
See you in a nother life Brotha!