I am beginning to think Ben has something to do with the sickness and for the following reasons
1. In "Dead is Dead" when Ben was leaving Danielle's tent, Danielle accused Ben: "You're the one who infected us, aren't you?". 2. Ben's mother died while giving birth to him which is what happens to any woman giving birth on the island. 3. He would have been at the temple at the time the French team went there and got infected. 4. Ben got cancer when other people on the island are getting healed. 5. Ben seems very dedicated in trying to find a cure for it so maybe he feels guilty for being responsible for it. Maybe this has something to do with why Richard was so set on accepting Ben as other when he was brough to him?