It is a mysterious sickness believed to exist on the island. Rousseau killed the other members of her team because she says they became infected.
Dogen, the Japanese man and apparent leader of the Temple, said that Sayid has become infected, or claimed. Dogen says it is growing inside him and will spread, eventually reaching his heart at which time all that he is will be consumed. Dogen further reveals that Claire has been infected just the same.