The first lostie "claimed" by the sickness by JR

First let me say the writers of this show are brilliant. As we progress through the final season, the concept of being claimed, or becoming infected, will play the most important theme of the show.

Recent theories posted on this site have posited the following criteria for becoming infected, or becoming "claimed":

1. People who are claimed die on the island.
2. Their bodies appear to revive.
3. They retain memories of their former selves.
4. They are under the influence of an evil force.

Based on what was presented in "What Kate Does", both Sayid and Claire have died and been claimed by what we assume is the MIB. We are also lead to believe this is what happened to Rousseau's science team. Recall in the season one episode "Solitary", Rousseau claims she had to kill her former team because they were infected and she could not let them leave the island. Apparently leaving the island after becoming infected (claimed) would not be a good thing. I believe this is the event that needed to be prevented in order to "save the world." This is ultimately what Desmond was preventing by pushing the button every 108 minutes. Unfortunately for the world, his failure to push the button the day Kelvin died allowed the crash of Oceanic flight 815, which in turn lead to a series of events culminating in the infected one leaving the island and eventually bringing about the end of the world.

Who and how this happens has already been shown and was always there if we paid attention. In season one, shortly after the crash, Claire told Jack she had not felt the baby kick. This is because the baby was killed during the crash. It was at this point that baby Aaron was claimed, or became infected. When Claire felt her baby kick after eating Jin's sea urchin, it was no longer the baby she had been carrying. Rather it was the being who would leave the island and in some way cause the world to end. For reasons unknown as of yet, MIB appears to be in favor of allowing this to happen. Jacob appears opposed, and hence their game.

Jacob's arranging for our losties to travel back in time to try and prevent the crash would appear to be a counter move in this game. Preventing the crash negates allowing the infected Aaron from ever be exposed to the world. Failure dooms us all.