A few of my opinions, so might not be correct:
- We have barely seen the others so of course we haven't seen them inject themselves. As Cappy alludes to something similar above, if Ethan knew Claire was going to get whacked, he really had no reason to lie to her when he said that they had enough serum for Aaron but not her. I think Ethan was really getting to like Claire, or at least drugged up Claire. Remember, bad guys always tell the truth about their plans/situations when they think the good guys are going to die.
Just because Claire was drugged doesnt mean she was euphoric all the time. She didnt react too well to Alex's actions, did she? Ethan may have been telling her what he thought she wanted to hear. Regardless, his line is far from proof that the sickness is real; Alex made it pretty clear that Claire was going to die.
- Rousseau didn't get sick because she shot the rest of her crew before she got sick. We have no idea how people even get sick or whether it is contagious. Anyway, as I have mentioned various times before, I really do believe Rousseau is telling the truth. I may be wrong, but I think if Rousseau was an Other, they would have shown us (the fans) her meeting with them, etc. Everything they have shown backs up the theory that she was in the exact same position as the 815 folks, had her daughter kidnapped and has survived for 16 years trying to find her daughter. She told Sayid that Henry would lie about everything, so methinks that she has had more run ins with the Others than we know.
Rousseau has never described what the sickness was, not to mention the fact that she has already lied to the survivors. The fact that she said they got sick is not proof that there is a illness on the island.
- Why would the orientation film mention the serum? Since we saw that it was Dharma Theatrical Glue, it seems pretty apparent to me that the orientation film was put together by the same folks that are the Others. Not every Other is Dharma employee (like Alex and the other kids), but Zeke probably works in the same team as the guy in the film. I think Desmond got duped and the hatch he was in is all part of the setup.
Thanks for making my point. If Desmond was manipulated into pressing the button, why couldnt he been manipulated into believing there is an illness?