15 Unanswered Lost Questions That Totally Have Explanations

Viewers first hear about a mysterious sickness in Season 1, when Danielle Rousseau mentions it to Sayid. She tells him she shot and killed her lover, Robert, because he was "sick", as was the rest of her crew from the Black Rock. This is brought up again when Rousseau says that Aaron might be sick.

So what was going on here? Was there ever a real sickness, or was Rousseau just crazy?

The Explanation: Well, there was no real sickness, exactly, but this wasn't just Rousseau being nuts. It's revealed that the Man in Black, a.k.a. the smoke monster, has the power to infect people and cause a darkness to grow inside them. That's what the sickness is: being infected by the smoke monster and being more receptive to its influence.

This is what happens with Claire in Season 4, lured into the jungle by Christian Shephard, as well as Sayid in Season 6.

The sickness is such a prominent threat on the island, in fact, that the Others developed a test to determine if a person is infected. They administer the test to Sayid, and when he fails it, indicating that he's "sick," they try to kill him. That's how afraid they are of people who are infected by the monster. 

But both Claire and Sayid end up redeeming themselves, so even when "sick", the person isn't totally beyond saving.