Some may call "What Kate Does" a pretty uneventful Lost episode. I somehow agree, but in most seasons of the show, the first episodes act as building blocks, establishing things that will either prove critical in the end game, or help us understand what is going on. So, we now know (or at least we think) that the no-crash timeline is a huge what-if scenario, complete with elements from the post-crash timeline. We now know of an impending showdown between whatever's left of Jacob (and that's definitely not Sayid) and the Man in Black. And, we now know that the infection--the same on Rousseau was talking about many seasons ago--will play a huge part this season.
Judging from Dogen and Lennon's reaction to Sayid's apparent resurrection, the infection has been around for quite a while, and has been busy wrecking havoc way before Rousseau found herself on the Island. They know it's the Smoke Monster's fault. They sure know a more dignified way of defeating the condition: better a pill full of poison rather than gunshot wounds to the chest. And they know that if the infection consumes a person fully, they will cease to become who they are.
It also goes without saying that the infection doesn't just attack at random. Rousseau wasn't infected; she just got crazy. So why Sayid, and why Claire? Sayid was probably exposed after Dogen's attempts to heal him: the spring where he drowned, after all, wasn't clear, presumably a nod to Jacob's death. Claire probably got it after following Christian, but that's presuming he isn't who he is, but is actually the Man in Black in disguise, much like fake Locke is to Ben.
While the pre-crash stories of the two don't exactly match up--Sayid was an Iraqi torturer, while Claire tried everything to rid herself of responsibility over her baby--there is something that ties them together: they were both torn between doing the right thing and doing what's convenient for them. They sure turned out differently, but deep in there, it's one and the same.
I expect the infection to claim a couple more survivors in the next few episodes. Ben is a shoo-in: Jacob's murder was a result of his doubts, more than fake Locke exploiting the loophole. His past was also full of these what's right vs. what's convenient dilemmas. I'll go on a limb and say Sawyer will be infected too, keeping in mind his past as a con man, and especially the way he dealt with Cassidy.
The rest simply aren't eligible for infection. Jack may have done questionable deeds but he's out looking for others. Kate may be a fugitive, but remember that her original crime--killing her biological father--was in an attempt to save her mother from abuse. Jin was just forced to do Mr. Paik's bidding. Hurley could do no wrong, and the same goes for Miles.
What's the end game, then? The season premiere hinted at a showdown between Jacob and the Man in Black--one between good and evil, some say--and the infection could be a way to delineate who goes on what side. The first episodes will see the survivors run away from the infection: some will inevitably get infected, and some will be safe. Those who were infected wouldn't necessarily go evil: they'll just lose their inhibitions, start relying on more drastic tactics, all in a bid to get what they want. (The similarities between Claire and Rousseau are painfully obvious.) And then it's the final face-off, with the survivors on both sides serving a higher purpose: keep the world in balance.
Unless, of course, there's a whole layer of deception that we have yet to uncover. I have to remember: these episodes are building blocks.