"So I’ve never before responded directly to the fandom’s comments about an episode, and I don’t plan to make a habit of it, but I couldn’t resist dropping in a thought about the episode “Yellow Fever.”
Which is this:
Dean is not a dick.
None of the writers, or anyone on the creative team of Supernatural, think Dean’s ever been a dick, past, present, or future. He’s a hero. Dean did NOT contract the ghost sickness because he’s a dick.
Victims contract the illness because they use “fear as a weapon.” Dean asks Lilith at the episode’s end, “why did I get infected?” And she cryptically responds, “you know why. Listen to your heart.”
We, as the writers, probably should have emphasized this mystery more, I take responsibility for that omission. But the point is: the reason he was infected is because of a SECRET he’s keeping. A dark secret that will be revealed in Episode 10. And not at all because of any dickishness, implied or otherwise.
--Eric Kripke, Creator Supernatural
Source: here and here (The Tribune article DOES INCLUDE FUTURE SPOILERS not included under my cut or in the first link.)