My assumption, once I saw the episode with the major Ben flashback, the one in which he kills his Dad and the other Dharmas, was that the gas used was what killed Danielle's crew. And that was also why the Dharmas still in the Pearl thought it necessary to wear protective gear to go out. But, I haven't tried to work out the timeline on this, to see if everything fits. However, Danielle has been there 16 years, and it certainly seems like Ben could be about 16 years older than when he participated in the purge.
That's a cool theory about Rosseau being a member of the Black Rocl crew, I'd never thought of that! Also, I had never made the connection between the chemicals used by Ben in the Purging and the "disease" that supposedly killed Danielle's team members and that Desmond's predecessor in the Hatch felt he needed to take precautions against. I still think they are two seperate issues, however. Also, don't forget that Daniel and his team brought biomat suits to the island as well. Jack found them on the ground after Daniel had parachuted from the helicopter and asked him why they were necesarry, and Daniel got nervous and made up an excuse about them being standard fair on the chopper, clearly a cover up. This makes me think that there is something more going on than the chemicals that Ben released if these new visitors are wary of the disease. Any other theories on what it could be?
I think Az-Phil was joking, since it has been established that the Black Rock has been there a lot longer than 16 years. It was a slaving ship that disappeared in the 1800s. Not only that, but the Lost Experience internet game, which the producers of Lost have deemed canon (i.e., true backstory about the island, the Hanso foundation, and the Dharma initiative), seemed to indicate that Alvar Hanso's grandfather may have been captaining the Black Rock--he did at any rate own the company that owned the ship.
As for why the freight people brought masks, it's possible they also brought gas, so I'm just thinking they were planning on using it for some reason. Or perhaps they'd heard there had been sicknesses on the island, but weren't aware that they had since cleared up.
well, it has been mentioned here and there like the otehrs have said above, the gas masks, protective garments, rousseau mentioning it to claire, the vaccines etc....but we dont know what it really is or what caused it. was there even a sickness really or was is rousseau who really went crazy and shot them all? perhaps it was something in the atmosphere after the purge? or the water?