The conclusion is "Mental states---> vulnerability of immune system". E show us that infections don't stem from disorder of the immune system, this is another story of disorder of the immune system, and has no any relationship with the argument.
E is so wrong. Let me give you a simper example: sleepiness --> loss of concentration; hunger--> loss of concentration; Either of these two is not the basis to dispute the other. In another word, we cannot say that because sleepiness causes loss of concentration, hunger must not cause loss of concentration. Unless there is a exclusivity clause in the argument, such that only one reason cause loss of concentration.
You misundertood E. If E said that the disorder of immune system in this group of people stems from higher level of exposure, it is the right answer.