I just noticed… In Yellow Fever, Dean gets the ghost sickness because he uses fear as a weapon. Sam explains it as them being hunters, basically (“Dean, all we do is scaring people”), but in fact he himself doesn’t catch it. Moreover, we discover later that the victims were murders, or implied in a murder.
So I thought, this is the season where Dean’s just gotten back from Hell, right? So could it be that Dean became infected because he tortured souls in hell, and the spirit considered him guilty?
This is a super depressing thought because Dean was a victim, too :( but. It fits.