If you don't want potential spoilers, don't read past this point. 🙂
Things we have seen in the show:
1) LOST has presented that psychics exist in their universe in the episode "Raised by Another". And in "All the Best Cowboys Have Daddy Issues" Walt demonstrates an amazing ability for always rolling the dice he wants for backgammon, which hints that there might be telekinetics as well.
2) Danielle Rousseu says that her crewmates were infected. They started acting differently. She had to kill them because she feared what would happen had they been rescued and the 'infected' ones got off the island. She never says it was a physical disease.
3) When Locke was dragged into the hole in the Season 1 finale, and they freed him, a cloud of self-motivated black smoke came out of the hole. It was the first visual evidence of something other-world like on the island.
4) The creature in the forest, that knocks down trees, causes damage but has not been seen by the viewing audience.
5) Both Charlie and Claire have a loss of memory when they are recovered from The Others.
6) Ethan Rom was able to pass himself off as a passenger of the plane, but did not give a name that was on the passenger list.
7) The island has strange magnetic properties. When Lock gives Sahid his compass, it points in the wrong direction. Sahid tells Jack that he thinks Lock tampered with it, but after the beginning of season two we know better.
My guesses based on what we have seen so far:
I think that the DHARMA Initiative, in experimenting with the island's unusual magnetic properties, opened a portal to someplace else. Something escaped. That something can possess people, and is the 'infection' that Rousseu fears. I think the DHARMA Initiative cemented in the room where the experiment happened as a stop-gap measure, and set up the human guardians re-reinitialize the equipment that keeps the portal closed. If the portal re-opened fully, the world as we know it would end.
I think 'The Others' are possessed people. Some might be people stranded on the island by other accidents, but I bet a chunk of them are the replacement teams send by DHARMA who never made it to the research facility. I think that the entities doing the possession can access the memories of the host; and that the body that was Ethan Rom was a passenger, and had memories that could be used to pass the possessing intelligence off as a passenger.
I think that the creature in the woods is non-physical. Under certain circumstances it can interact with the physical world, but it isn't material in the sense that we are material.
I think the unusual magnetic properties keep it from being mapped by satellites. Perhaps there is even a mirage effect, so that the island is never spotted by sailors. The only people who find it are the ones to get too close. If the DHARMA project was run by scientists intent on studying the paranormal they may have set out to find a place like the island by searching satellite and ship records looking for anomalies that might be hiding a place worth studying.
Just a few thoughts. Most likely everything I think will be proven wrong next week. 🙂
ps. Damn! I thought I had published this!
One other thought I have had... the 'Others' that plague the survivors fromt he tail section are not the same 'Others' that took Walt. Those spoke, had a boat, guns and a bomb. They were tool users. They don't seem the sort to slink around in the forrest bare foot. I think it goes without saying that just because the folks on the island think they are the same group doesn't make it so. This show is all about turning your ideas around on you. I think that's why I like it so much.