When Dogen used the term "claimed" to describe Sayid's condition, several things came to mind. First, possession by an outside force, such as an (evil) spirit. Second, a more scientific explanation, like an infection. Rousseau claimed to Sayid that all the Others were infected by the "sickness." And we learned she was referring to the transformation of humans after being captured (consumed) by Smokey. It is also of note that when Desmond was caged up in the Hatch, he was routinely giving himself a "vaccine" against an unknown illness. That is why the Island was under some kind of quarantine.
In a pseudo-scientific explanation of the Dogen analysis could follow the pattern of a supernatural staph infection:
Staph bacteria are normally found on the skin or in the nose of about one-third of the population. If you have staph on your skin or in your nose but aren't sick, you are said to be "colonized" but not infected. Healthy people can be colonized and have no ill effects. However, they can pass the germ to others.
>>>> This is what could have happened to the 815ers. Once they were "brought" to the Island, they have been in contact with the Island bacteria and "colonized."
Staph bacteria are generally harmless unless they enter the body through a cut or other wound, and even then they often cause only minor skin problems in healthy people.
>>>> More proof that you are safe unless you have an open portal for the Island bacteria to invade a body. Prime example: Sayid's gunshot wound in the belly.
A weakened individual can have the bacteria grow into a serious infection. The bacteria could be immune from antibiotics as over time the germs find a resistance to the drugs. Also, germ mutations can form making current antibiotics ineffective.
Smokey can have the pattern of a mutant Island bacteria colony. It can infect human bodies when they are in a weakened state, such as just before death. Example, during the freighter fire-fight, Claire's cabin was blown up, and she came back to life in a groggy state (just like Sayid at the Temple). Dogen claims that Claire is like Sayid, infected by the darkness that will spread to their heart, obliterating that person as you would know them (like Flocke). Whether this Smokey bacteria can be a parasite and use dead bodies like Christian as a reanimated host is unclear. There may be two different minute cellular organisms at work.
The bacteria can only take hold with a weakened individual. In an Island sense, that may mean an individual whose mental or moral state has been weakened to a state of collapse and abandonment of hope. Sayid's conversation to Hurley at the van was a good example: Sayid was resigned to his fate because knew his time was up, and he knew he was going to hell ("an unpleasant place"). But little did he know, he would have a second chance on the Island to become a different kind of monster.
