"I'm going to tell you something that might frighten you," Patty Duke remembers her psychiatrist saying.

"I believe you're a manic depressive."All she felt was relief.

"The nightmare is shared by millions of Americans.

They suffer that psychiatrists formally call affective disorders and what most people know as depression.

And the nightmare is growing.

For some reason, depression is on the increase in the United States--not among older Americans, as might be expected, but among the young.

Today's baby boomers and yuppies are much more likely than their parents to have a depressive illness.

The most common kind is unipolar depression, whose patients suffer the blackness of despair and hopelessness.

Other patients, like Duke, have manic depression, called bipolar disorder. Still others experience a continuing, but not crippling, mood depression--dysthymia.

For Duke, who will be 42 on Dec. 14, the nightmare lasted more than 20 years.

She swung from periods of deep depression, in whcih she fought off suicidal feelings, to episodes of destructive euphoria when she chartered private jets and went on shopping sprees with money she didn't have.

Meanwhile, gossip columns hinted at drug or alcoholism to explain her wild behavior.

Depressive illness has been headline news lately.

Mike Tyson, the heavyweight champion is or isn't manic depressive, depending on which os his psychiatrists you believe.

Sukhreet Gabel, the central witness in the Bess Myerson trial, has had depressive illness since adolescence.

Depressive illness is a disease of the famous and the obscure.

It has afflicted writers as disparate as Ernest Hemingway, F. Scott Fitzgerald, Sylvia Plath, Edgar Allan Poe and Lord Byron; such musicians as Peter Tchaikovsky, Gustav Mahler, Hector Berlioz and Gioacchino Rossini, and world leaders like Abraham Lincoln and Winston Churchill, who called his recurrent bouts of depression his "black dog."

And going by the averages, at least one of every 25 people reading these words is afflicted by the condition.

Surveys show that about 5 percent of teenagers have depressive episodes.

In the population at large, one of every four women and one of every eight men will have a depressive episode at some time, experts say.

Many of those depressives don't realize they're ill.

Like Duke, most sufferers go for years without knowing they have a real, treatable illness.

Many never go to a doctor for help; if they do, the doctor often misses the diagnosis.

And for those who get treatment, there's a final hurdle: the stigma attached to mental illness.

That stigma even played a role in the presidential campaign.

Michael Dukakis had to vehemently deny a report that he once needed counseling for depressive feelings.

The stigma makes most patients reluctant to talk about their problem.

A Manhattan executive--call him Richard Stevens, which is not his real name--recalls what happened when he told his boss he had been treated for depression.

"He was very liberal and open-minded, but he just didn't understand what I was talking about," Stevens said. "A woman I worked with was having chemotherapy for breast cancer. She was given a cot in her work area so she could rest. My illness was psychiatric, so I was treated like a pariah.

But now there are signs of change.

After years when public relations people covered up for her, Duke has gone public, telling her story "because I want to help people who are suffering the way I suffered" and sponsoring a treatment program in a Palm Springs, Calif., hospital. Her career hasn't been hurt by it, she says.

And the National Institute of Mental Health has started a nationwide program to make people more aware of depressive illness.

Judd says it's "the first science-based program of detection and treatment in the 100-year history of the institute." It's called D/ART, for Depression/Awareness, Recognition, Treatment, and is designed to make patients and doctors recognize the symptoms of depressive illness so it can be treated.

"It's a good place to start to change public attitudes toward mental illness," said Joyce Lazar, the head of D/ART.

The major reason, she says, is that once diagnosed, depression is a highly treatable disease. At least 80 percent of patients can get relief in a matter of weeks from drugs, psychotherapy or both. Most of the others can be helped by a variety of other treatments, including electroshock therapy.

Duke, for example, now regularly takes lithium--a standard drug for mania--and hasn't had an episode of depression or mania in years.

Stevens was helped out of two bouts of depression by antidepressant drug therapy and took lithium for a manic episode. He has been working and living normally for years.

Stories of successful treatment of mania and depression can be multiplied manyfold--which is what D/ART is counting on.

"When there's greater recognition that depression responds to treatment, attitudes will change," Lazar said. Her team is trying to teach people that there's a point at which normal depression, the blues or the blahs, becomes a condition that requires treatment.

Almost everyone is depressed at one time or another, usually for an understandable reason--a personal loss or setback. Depression can be seasonal; some people suffer from it regularly during the dark winter months. Stress, too, can bring on depression.

The stress of Christmas time is a major example.

Usually, this kind of depression goes away.

Sometimes it doesn't.

What doctors call "clinical depression"--the kind that requires treatment--often begins with the kind of everyday depression that should go away with time but doesn't.

Duke's case is typical.

Her first depressive episode occured when she was 19.

It came when she was undergoing several stresses; a new TV series, a new marriage, the death of a father figure.

She was hospitalized and given antipsychotic drugs for eight weeks before returning to work.

Perhaps 70 percent of people who have one episode will have a recurrence. Duke had a manic episode three years later, then a series of at least 10 cycles between mania and depression that ended only in 1982, after 17 years of illness, when the psychiatrist she was seeing finally diagnosed her condition correctly.

With Andrews, a New York executive--again not a real name--the trouble began with a manic episode where "I thought people were out to get me," she said. "I couldn't sleep, I just talked through the night to my husband. My father found a psychiatrist and he diagnosed it as paranoid schizophrenia."

Her euphoria was followed by a period of deep depression--"not getting out of bed, not even combing my hair or brushing my teeth, just lying there.

There were a couple of times when I thought about ending my life."

A friend with manic depression made the right diagnosis. Andrews has had recurrences of both mania and depression since, but they've been manageable with antidepressant drugs and lithium.

* The National Depressive and Manic-Depressive Association has local support groups that hold periodic meetings.

Telephone numbers and addrresses of local groups can be obtained by calling the non-profit association at (312)939-2442 or writing to National Depressive and Manic-Depressive Association, Merchandise Mart, P.O. Box 3395, Chicago, Ill. 69654.

Information about the National Institute of Mental Health's Depression/Awareness, Recognition, Treatment (D/ART) program can be obtained by calling (301) 443-4513 or writing NIMH Public Inquiries, 5600 Fishers Lane, Room 15C-05, Rockville, Md. 20857.

* This is the season for depression.

Holiday depression is a problem for millions of Americans, psychiatrists say.

"Lots of people have mood swings around the holidays," said Dr. Gerald Klerman of New York Hospital's Payne Whitney Psychiatric Clinic.

* Defining the terms of depression:

--Affective Disorder--A general term for any of the depressive illnesses.

--Antidepressant Drug therapy--Treatment of depression with one of several medications that cause improvement by altering brain chemistry.

--Antipsychotic drugs--Medications used to treat schizophrenia and other mental disorders.

--Clinical depression--A depressive episode which requires medical treatment.

--Depressive illness--Disorders that affect mood, including depression, manic depression and dysthymia.

--Dysthymia--A less severe, chronic form of depression. Symptoms are less disabling than those of major depression.

--Lithium--A metallic element that has been found effective as oral medication for mania and can also be used for depression.

--Manic depression--An affective disorder characterized by cycles of elation and depression. Manic cycles can cause irratibility, insomnia, grandiose thinking, non-stop talking, paranoid feelings and other inappropriate social behavior. Also called bipolar depression.

--Major depression--An effective disorder whose symptoms include persistant sadness, hopelessness, loss of pleasure, apathy, insomnia, loss of appetite, headache, digestive disorders and chronic pain/ Also called unipolar depression.

--Paranoid schizophrenia--A mental disorder characterized by delusions.

* Doctors diagnose depression when four or more of these symptoms persist for two weeks:

--Continuing sad or empty mood.

--Feelings of hoplessness, pessimism.

--Feelings of guilt, worthlessness, helplessness.

--Loss of interest or pleasure in everyday activities.

--Loss of interest in sex.

--Sleep and eating disturbances.

--Restlessness, irritability, fatigue.

--Difficulty concentrating, remembering, making decisions.

--Thoughts of death or suicide.

--Physical symptoms or pain non-responsive to treatment.

* Symptoms of mania are:

--Inappropriate elation.

--Grandiose notions.

--Increased talking, moving, sexual activity.

--Inappropriate social behavior.

--Insomnia.

--Inability to make logical decisions.