the illness (spoilers thru S4 or thereabouts)

[Mr Light]

Aside from what Rosseau claims, this would seem to be the most likely story:

Danielle arrives on the Island in 1988 as part of the Dharma Initiative. She manages to survive the Purge, does not convert to the Others, and flees to live in the jungle. Just as Kelvin and Radzinsky survived the Purge by living in solitude, so does she. Her story of having to kill the other members of her team? They converted to the Others and she fought them to escape, but they kept her daughter. Over the intervening 16 years of solitude she goes insane and her delusions of her past become reality to her.

 

[Jenee]

Danielle could easily have been part of a second Dharma science Team, sent to find out what happened after the purge.

The Purge happened several years after Danielle's team arrived. She said she'd been on the island 16 years, and Horace told Locke that the Purge happened 12 years ago.

Huh. Then the sickness would explain why the inside of the hatch said 'quarentine'. Why they had to put on protective gear to go outside - sure, Kevin realized it was safe (at some point), but obviously, before that, they considered the threat to be real.

 

[Mr Light]

My interpretation of "Quarantine" is that it was a ruse to force the Swan employees to stay in the hatch and keep pushing the button, since the world would be swallowed by the micro black hole if they didn't. Just like the Pearl employees were mislead about their observances being used.

The real question is a) why did the Arrow have a Quarantine sign when it was just math-forecasting and b) was the Quarantine ruse devised after the Purge to keep them out of the Hostiles' hands or beforehand?