During the first season, Rouseau said that her team got sick (16 years earlier). Spray-painted on the inside of the Hatch was the word "Quarantine." Desmond, Kelvin, and all the rest of the Others seem to be injecting themselves with either a vaccine or antibiotic drug.

I think that sixteen years earler, there was some sort of a plague on the island, possibly a virus created by Dharma that affected only the human beings on the island (this was hinted at, I think, in the Lost Experience that ran during the second season). Dharma then came up with a vaccine (and maybe a "booster" shot you have to take every so many days) to protect the people on the island from the plague.

I think the pregnancy thing is an unintented side effect of taking the vaccine.

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charlie98210
"Whoa!" she said. "Cry 'Havoc!' And let loose the dogs of war!"
And, like Banquo's ghost, they just won't go away.