I made a brief post about this on the Sundown Talk page, and just thought I'd elaborate it further. Chances are it's probably not right knowing Lost but there's no harm in guessing :)
The first thing to mention is Candidates seemingly can only be killed by other Candidates. This is why Dogen gives the poison to Jack to give to Sayid, and why Dogen doesn't kill Sayid in Sundown when he had to chance to. Dogen is probably not a candidate and Jacob told him not to kill any candidates if he wants his son to survive. But what would happen if a non-candidate kills a candidate? What happens to them?
Personally, I think if a candidate is killed by a non-candidate, MIB has a chance to infect them. He brings the candidate back to life, and when he gets an opportunity to talk to them, they become fully infected and serve for MIB. We see this with Sayid, he is mysteriously brought back to life, and appears to be fine until MIB promises him anything in the world. This would mean that Roger Linus is not a candidate, or perhaps he was crossed off before he shot Sayid.
Claire is another person who is infected. But where/how did she die? There were a lot of theories she 'died' in the house explosion in The Shape of Things to Come, and I think they were right. She was 'killed' by a random mercenary who was never a candidate, MIB quickly brought her back to life. Afterwards, she wasn't feeling very well, Miles gave her funny looks, and then she sees Christian (MIB?) in the jungle and the next time we see her, she's gone all weird in Jacob's cabin. Christian/MIB talked to her and she became infected.
Who else has become infected? Well, the French team did. We have no idea what happened to them in the Temple, but a loose theory is that some non-candidate killed all of them and they were resurrected, where MIB came back as Montand and infected them.
One final point to make, is Colleen's funeral in The Cost of Living. http://www.youtube.com/watch?v=V6qZR2JGdkI White shirts = some kind of allegiance to Jacob, and the way they set her body on fire and released into the ocean is bizarre. We know the name Pickett was on Jacob's wall (which could be Colleen or Danny) and she was killed by Sun. Perhaps Sun isn't the 'Kwon candidate' or at least they weren't sure which Kwon is the candidate, so to make sure MIB never resurrects Colleen and infects her, they set her body on fire and release it into the ocean? Perhaps Jacob taught the Others this funeral method as a precaution in case MIB infects anyone? Anyway, I've blabbed on too long, hoped you enjoyed reading :)