You are mistaken about depression not having observable effect on the body. I wrote a post about that; it's a direct sibling to your original post.

Your point is a bit conflicted. Like this person pointed out, it seems that you're saying that it's serious and not that serious at the same time.

However, I think that the observation you wanted to convey wasn't about the comparative seriousness of depression, but rather that it's in some sense in a different realm than for example AIDS or shingles, even if the realm of some of its effects are similar (which you argued against, but you're wrong). And, I sympathize with that. Depression does seem to be made of a different cloth, but it's quite hard to elaborate further, and I won't get into it here.

It is odd to mention shingles in this context, as depression is a very common symptom of herpes infection, actually fixed by antiviral medication.

So, even on its own terms, the argument is incoherent and, not to put too fine a point on it, wrong.