I'm pretty sure Ben knows that Rousseau is alive.
I'm convinced that the "sickness" doesn't apply to women who conceive off the island, which would explain Ben, Alex, and Aaron.
oh, and by the way, I don't know if this has been posted before, but did anyone ever see "Gia"? It was an HBO movie in 1998 about a supermodel, featuring a famous lesbian scene with Angelina Jolie. What a lot of people don't know is that the other chick was... Elizabeth Mitchell, who plays Juliet.
Here's the NSFW video