GMAT Critical Reasoning (CR) | Re: A study comparing a group of chronically depressed

I agree C is correct..but I don't understand why E is incorrect. E clearly weakens the argument by saying that there are other sources(high exposure to infections in this case) that can cause disorder to immune system and it need not be primarily because of the mental state.
As far as C is concerned, it weakens the argument by stating the opposite of what's stated in theconclusion.

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