"What would have happened if we were rescued?" by Ricardus Alpert

After Last night, I believe that the sickness is far more important than we might have believed. IMO, MIB is the cause of the sickness and it's very possible that it's a method for him to control people. Given how afraid Dogen seems to be of it, I believe it's contagious. Personally, I believe this is the reason Jacob has been keeping MIB/ The Monster on the island. Because he knows that if MIB escapes, he'd infect the outside world and the consequences for that won't be pleasant at all. A few days ago, I theorized that The Others' purpose is to act as The Monster's "Prison Guards." We now know why. The Monster is capable of infecting everyone on the outside world, and probably intends to do so. It also explains why The Others show distrust of outsiders, sometimes torture them, and take their blood samples. It's all in order to see if they are "Infected."

It was even foreshadowed as early as "Solitary"-

DANIELLE: He was sick.

SAYID: Sick?

DANIELLE: It took them, one after the other. I had no choice. They were already lost.

SAYID: You killed them.

DANIELLE: What would have happened if we were rescued? I couldn't let that happen. I won't.

Suddenly, it all makes sense.