I've been thinking for a while about the concept of "Life Energy" and how that might apply to The Temple, The Sickness and how the Island Heals.
Lets suppose that latant an the island is a virus, a virus that not only reproduces itself but that also fixes human tissue. The main problem with the virus is that while it has a positive healing effect on most people (Locke , Rose) for some it has the debilitating mental effect (The French Team, Claire). To combat this, Dharma made a vaccine which weakened the virus and kept it in its healing state.
The virus probably originated from another pocket of electromagnetic energy, which I believe can be found underneath The Temple. This energy manifests as the heat under the pool, from where I think the virus ultimatly originates. In dowsing people in the pool (Ben, Sayid) they are attempting to exploit the energy's human-friendly effects. Unfortunatly for Sayid, this has left him exposed to the Virus, which is now beginning to have the debilitating effect. Ben on the other hand has lived on the Island for most of his life. He had built up an immuinty to the mental effects of the virus by the time he was exposed to the pool.
The hourglass is probably the recomended amount of time that energy exposure is required. Even when Sayis regained consciecesness, he hadn't had the required exposure that they foresaw. It was maybe this continued exposure after his minimum amount that left him exposed to the Virus.
Note that all of the tests Dogen used were to do with energy. He used electrical and thermal energy. Maybe he was testing the amount of residual life energy still left within Sayid. The amount of pain he suffered maybe showed an overexposure.
EDIT: I didn't mention earlier about two other aspects to the theory. The first is the impact on pregnancies. My thought is that feotuses conceived on the island are exposed for a long while to the virus, but are very susceptible to it. The fetus's mental state contradicts that of what the body expects, so the body isolates it and sees it as a threat. Of course, the body isn't used to getting rid of such a large invader, and both die.
Another aspect is the Smoke Monster (hereafter "The Entity".) The entity is primarily an energy-based life form, and can control the energy-affected particles present within the virus. The ash effects both because it contains a ferrous compound created by the energy on the island and has a negative polarisation to the energies. The Others want to stop the Entity not only for their own safety, but because if the Entity controls the large amount of virus in the Pool it may be able to extend beyond the island.
Now if we assume that in LOST energy can have a mental state, then the electromagnetic energy present under the pool may have a certain latant mind, although not as focussed as the Entity. Jacob, I believe, had been fully infected with the Virus but instead was being used as a host for the mental state of that energy. Now that Jacob is dead, the entity is angered and has less control over its effects.