
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.