Paula Abdul Alhazred from the Totally Amazing LOST Blog of Awesomeness makes a very good point regarding the Sickness in the comments from my last review:
I think this episode sort of retroactively answered a lot of questions about the Sickness. Obviously it has something to do with the black smoke, and I would guess the DHARMA vaccine protected people from the negative effects of exposure to the island. (The vaccine was labeled with CR, and the C probably stood for Cerberus). Ethan must have been injecting Aaron with the serum in order to protect the fetus from the influence of the monster. I really like that just by answering one or two questions, we can extrapolate the answers to many other questions from previous seasons.I like this theory a lot, especially since it does tie a lot of stuff together from the early seasons. And if Smokey is the one causing the Sickness, it also explains why none of the survivors ever got sick - the Monster, by-and-large, really never came into contact with most of them. Who has had prior exposure to the Monster before (first encounter)?
1) Locke (Walkabout)
2) Jack (Pilot, first seen in Exodus)
3) Kate (Pilot, first seen in Exodus)
4) Charlie (23rd Psalm)
5) Eko (23rd Psalm)
Hurley was part of the Exodus crew too, but didn't see the smoke then. Aside from that, Smokey really never contacted many of them. So the question now is, if DHARMA really did come up with a vaccine to the Sickness, how did Claire get infected since she was one of the very few who actually received the vaccine. Furthermore, how the heck does the vaccine work? Do most of the Others have anti-Monster antibodies in their blood?
This (along with the sonic fence and summoning portal under Ben's house in the Barracks) also suggests that DHARMA must have known a lot more about Smokey than we've seen on the show. Very cool.