Critical Reasoning

Can anyone of you explain to me how come this answer comes up? I tried to read the explanation on the book so many times but still cannot get it yet. If possible, compare with sth more personal pls. Thx.

Question No.7 from Verbal Review:
A researcher discovered that people who have low levels of immune-system activity tend to score much lower on tests of mental health than do people with normal or high immune-system activity. The researcher concluded from this experiment that the immue system protects against mental illness as well as against physical disease.
The researcher's conclusion depends on which of the following assumptions?
Answer: Mental illness does not cause people's immune-system activity to decrease.

The assumption of this question is "cause-effect" assumption. In this case the direction of cause- effect cant be reversed. If you rephrase the correct answer like this way-"immune system activity decrease causes mental illness" it will be clear to you. Try to understand this way.
thanks

This question still makes no sense...

Can anyone give further explanation?

Discovery: low immune system is associated mental illness
Conclusion: immune system protects against mental illness

Can you come up with an alternate explanation of why people with a low immune system have mental illness?

Alternate Explanation: mental illness lowers the immune system (not the other way around)

The researcher has to assume that this alternate explanation is not true.