INFECTION

One of the compelling plot points was the concept that the island had a mysterious "infection." People had to be given shots to ward off death.

Claire was told that she had to take shots in order to save her baby.

Desmond was told outside the Hatch was a hazmat zone, but he still had to take shots to ward off the infection.

The Others pregnant women continually died while in their third trimester, which some blamed on the island/infection.

Without a clear understanding of "what" the island really was, many people believe the "infection" story line was merely a ruse to control people.

But Juliet, who appeared at first to be an ethical doctor, gave Claire injections - - - if false, would violate her duty and oath as a physician ("do no harm to a patient.") Even a placebo that causes mental anguish as an intended result would violate that oath.

Perhaps, at one time, the infection plague was true. It was just passed down as a story, a myth, by the island natives and Others as a means of making visitors leave their island.

An infection is defined as the process of infecting or the state of being infected by a disease.
It is also defined as  the presence of a virus in, or its introduction into, a computer system.

The origin of the word is late Middle English: from late Latin infectio(n-), from Latin inficere ‘dip in, taint.'

The dictionary definition raises two points. The island infection could be a metaphor for people becoming "tainted" or infected by something, such as evil, if this was a place of the underworld, or judgment. The events on the island were merely tests of character and morality.

Another possibility is that the infection was actually a computer virus. Computer avatars would see a computer virus as a disease that could kill (delete) them (their program).  If you buy into the theory that LOST was merely a large MMOG, then the infection was one of those booby-trap hurdles one had to pass in order to level up to the next mission.

In any event, the infection angle showed a great deal of dramatic promise. But it quickly faded away without resolution.