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The Man in Black is a carrier of the Sickness
The Man in Black is a prisoner of quarantine, because the Island's healing properties provide (mostly) a vaccination effect against the virus / plague the Man in Black is carrying. If the Man in Black were able to escape the Island, this would result in a worldwide and lethal pandemic and make the possible reunion of the Kwon family, as suggested by Widmore, a "short-lived" one.
Physical contact with the Man in Black / Smoke Monster will result in contagion (e.g. Rousseau's team) with only those previously touched by Jacob or subsequently Richard (the Others) remaining immune.
The Sickness is a result of absorbing more of The Source
- In Across the Sea, Mother told young Jacob and young Man in Black that The Source of the Island is inside every human being, at least a little for everyone. Man wants more of it, even if they can't have it. The Sickness occurs if a human does, through some unknown means, bypass this by gaining even more The Source than they need or deserve. This means, hypothetically, The Source is the Sickness, and because the Man in Black presumably absorbed all of it (which is what made him the Smoke Monster), he is now the carrier of The Source and thus the Sickness as well.
- Enter Claire and Sayid: They have had direct contact and maniputation with the Man in Black. Again, presumably, because the Man in Black now carries The Source inside of him (because he entered the Cave with the Source in Across the Sea), he was able to give Claire and Sayid some of The Source, connecting him to them and allowing him to control them.
Danielle had the sickness
She looked similar to Claire did after she caught it.
The blast door map
The blast door map contains the statement: "Aegrescit Medendo". Translation: "The remedy is worse than the disease". This refers to death being the "remedy" to the "disease". The poison pill Dogan offers is worse than the infection.
The Sri Lanka video
The sickness is the disease referred to in the Sri Lanka video by Mittelwerk, i.e., "They think they are infected by a virus carried by local macaques...", "We must make absolute certain we are hitting precise genetic targets...", "We need not take any more lives than is absolutely necessary. Yes?", and most importantly "...we have engineered into the virus."
The Sickness is "Malevolence"
"The Sickness" is that "evil" that the Island keeps in line, some it sometimes "slips through the cracks".
A "Zombie" Virus
As we’ve seen, the sickness can bring people back from the dead and cause them to behave in an extremely violent and erratic manner. Vocalizations seem to be brought down to nothing more than simple sentences or complete silence. Former personality traits are lost and loyalty/emotional memory seems to fade away. Claire, Sayid, and the science team were all killed or “snapped out of it” eventually, but what if they hadn’t been? Perhaps a progression of the sickness would lead to them devolving into Romero-esque zombies. This correlates the writers’ constant jokes about zombies. This could also explain the extreme fear of a fairly easily preventable infection and explain why Ethan, who seemed somewhat mild-mannered in Juliet’s flashbacks, became violent and murderous after venturing out into the jungle to infiltrate the Oceanic 815 survivors. It’s also notable that the Ajira 316 passengers that supposedly killed each other had some wounds reminiscent of those that are fatal for a traditional zombie in fiction (ie: destruction of the head).
The Sickness doesn't exist
Rousseau and the other members of the science expedition were driven insane when Nadine and Montand were suddenly killed by something they as humans couldn't comprehend (Smokey). Sayid and Claire were manipulated by promises and lies. There is no true "sickness".