You know about clinical depression being a treatable physical illness, but does the person you care about know?  If not, you are faced with a problem.  Education is the key. ( Check NAFDI's bibliography for books such as Understanding Depression by Drs. Donald Klein and Paul Wender.)  The person afflicted has to gain an understanding of the illness to realize that there is no reason to be ashamed any more than they would be ashamed if they had hypertension, for example.  (Of course, the situation is different if you have reason to believe (s)he's suicidal; get hir to an emergency room immediately.)

You have to be aware that approaching the depressed person is not without dangers to your relationship. You've heard the expression, ‘Kill the messenger!'  However, you mean well and you are trying to improve the quality of your loved one's life and, possibly, even to save it.   The person you care about may recognize this and may even find comfort or relief in your support.

You can approach this person carrying a book like the one we have recommended while saying something like, "I was looking through this book and you wouldn't believe just how many of the symptoms in here seem to match what you're going through.  Why don't you take a look and see what you think?"  You might also suggest that the person see how many of the symptoms of the checklist match hirs.  It must be made clear that you are only doing this because you care and hate to see the person in so much pain and trouble.  Remember that your timing and tone are also important.  You would not want to try this when (s)he is particularly upset.  You will benefit most from a tone which is concerned but matter-of-fact versus one that is critical or distressed.

In the best of all possible worlds, your loved one would respond immediately. "I have almost all of these symptoms. I should call a doctor." More likely you're going to have to fight a war of attrition; constantly working on them and wearing them down until they are willing to go for help.  Remember, your goal is only to get them to a competent physician trained in the diagnosis and treatment of affective disorders.  In order to do this you'll have to work hard to overcome the still existing, stigma of depression while not overwhelming the depressed person with information.  Suggest accompanying them on at least their first visit to the doctor.  It may help to overcome reluctance and it's also a very important aid in proper diagnosis.

It's generally not easy!  It generally takes time.  It generally takes a lot of effort and patience on your part with no real assurance that, ultimately, you'll prevail.  But, if you care enough, it's worth trying.  In the meanwhile, it is very important to take good care of yourself.  You might even try visiting a support group that offers support for people in your situation.