Sayid, infected and murderous. ("Sundown")
Danielle holds Robert at gunpoint. ("This Place Is Death")
The Sickness manifested as a severe mental change on the part of the afflicted. One apparent sufferer, Robert, suddenly and uncharacteristically attempted to kill his fiancee. Claire post-infection became a mentally unstable murderer and cradled a boar's skull as a baby. Earlier jungle hermit Danielle Rousseau acted similarly, though she claimed to be the only expedition member to escape the sickness. Danielle herself killed all other members of her expedition and several other characters described her as insane. Sayid, after his infection, killed even more readily than usual, and he said he could no longer feel emotions.
Members of Danielle's team contracted the sickness after exploring below the Temple, an area the Man in Black had just entered. The smoke monster once similarly tried to drag Locke underground. Sayid contracted the sickness after dying in the temple spring. He resurrected soon after, and credited the Man in Black. Claire's moment of infection is less clear. ("The Little Prince") ("LA X, Part 2") ("Cabin Fever")
Interestingly, all individuals known to be infected have been so on the Temple grounds. Sayid was at the temple spring, and members of the Bésixdouze expedition were dragged into the Tunnels, within the boundary of the Temple. The only exception is Claire, because how or where she was infected is not known, but it is a possibility that infection can only occur at the Temple. ("LA X, Part 2") ("This Place Is Death")
Danielle, who thought the sickness was a literal illness, thought the Others transmitted it, calling them "carriers". ("Solitary")
Dogen tortures Sayid with a red-hot poker. ("What Kate Does")
The Others tested individuals for infection by taking their blood and torturing them. They subjected Sayid and Claire to this test. Dogen claimed it measured their level of good and evil.
Sayid redeems himself by saving his friends. ("The Candidate")
According to Dogen, infection was irreversible. Rather than trying to cure Sayid, he tried to get Jack to kill him with a poisonous pill. Sayid and Claire, however, both rebelled against the supposed evil within them. ("What Kate Does") ("The Candidate") ("The End")
List of sufferers
|Claire||At some point after the rescue of the Oceanic 6, Claire was brought to the Temple because she was infected.|
|Sayid||Sayid was diagnosed by Dogen by using a complicated machine.|
|Brennan||Infected according to Danielle Rousseau's account.|
|Lacombe||Infected according to Rousseau's account.|
|Robert||Infected according to Rousseau's account.|
The series referenced a number of sicknesses unconnected with the Man in Black's.
Staff doctor applying vaccines ("The Man Behind the Curtain")
In the 1970s, the DHARMA Initiative inoculated new recruits. Swan residents also used this vaccine, and the Swan mural contained the enigmatic phrase "I am sick." The Others later injected Claire with this vaccine, and several DHARMA stations warned of a quarantine. This vaccine turned out to be a placebo, and the quarantine was a deception to confine workers to stations. ("Live Together, Die Alone, Part 2") ("The Man Behind the Curtain") ("Namaste") A jigsaw puzzle reveals that Kelvin and Radzinsky believed "There is no sickness".
Pregnant women suffered complications unconnected to the sickness. Aaron briefly developed a fever that worried Claire about the sickness, but it cleared up after a few days. Claire later suffered nosebleeds thanks to an artificial implant. ("Maternity Leave") ("One of Us")
Regina commits suicide. ("Ji Yeon")
Time offshore close to the island caused what Captain Gault called a "heightened case of cabin fever." Brandon behaved erratically and died. Regina behaved listless and eventually killed herself. People traveling to and from the island sometimes suffered from Temporal displacement.