Or, according to the Lost Experience, during the Worldwide Wellness and Prevention Development Program - what was really going on with the organ harvesting was they were looking to see who actually contracted the virus. [Mittlewerk scusses taking "radical action" and killing innocent villagers under the pretense that they are bringing a cure to a disease carried by macaques. In reality, they have engineered into the virus, and are purposefully testing it. A 30% mortality rate will help them solve the equation by altering one of the core values, extinction through over-population. They require all the bodies of the dead for full genetic research because they only want to kill off thirty percent of the world's population to save the remaining seventy percent. ] - The Lost Experience
Apparently some people are naturally immune to the virus (if you think the virus and the "sickness" are the same) which might explain why Rousseau survived and why Jack is terrible in his flashforward.

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