The deal is that the women on the island can't get pregnant. That means their population can't grow. That's why they were stealing children and looking for the "good" people.
That is also why they were testing #^@! on Claire while she was pregnant. That's also why they made Juliette come to the island in the first place, so she can help the women get pregnant or understand why they don't get pregnant.

I was wondering about the cancer thing. Ben said that there was no cancer on the island, but how did he grow a tumor on his spine?

And I can't believe they have spies all over the place. How they spied on Juliette's sister and all. It's freaky.

One last thing and i'll shut up. The way Sawyer looked at Kate when she came back