Is it Mental or Physical?

That's often the question that comes after I say to my love (or he says to me), "I don't feel good."

Sometimes, it is hard to tell.

Nausea caused by anxiety.  Or nausea causing anxiety?

Feeling depressed because of being worn down by a cold.   Or just happening to have a cold and being depressed at the same time.

Can cold viruses cause depression or other mental changes?  I don't know... Sometimes I think so.  But it has been pointed out to me that people don't usually feel good when they are sick.