(FORTUNE Magazine) – THE MARCH of science has produced this arresting tidbit: Though most of us are in a blah or foul mood three days out of ten, an annoying 0.5% of the population is in a good mood all the time. And just your luck, one of those happy-go-lucky types works in the adjoining office. Bill is perky. Bill is chatty. Bill, in fact, is getting on your nerves -- more and more so, all the time. And if Mister Happy Face slaps you on the back one more goddamn morning and bubbles about work, he's gonna get a fat lip. Except lately you haven't had the energy to bust anybody's lip. It's a colossal effort to drag yourself out of bed. Bill and everybody else make it impossible to concentrate. The headaches and lack of sleep just make it worse. Weekends are no better. Golf used to be fun, but it turned into such a useless, boring game. Sex is a chore. Work stinks. Home stinks. What to do? Bark a few orders to the staff to show them who's boss, close the office door, and hope no one notices you can't get anything done? Jump on a plane and hide out touring the regional offices? An alarmingly pleasant thought flashes by: ''Maybe this misery will end if the plane loses a wing and . . .'' Warning signal, warning signal: Pal, it sounds as if you've got more than the ordinary blahs or even burnout. Quite possibly, you are in the throes of a very common illness: major depression. Because it masquerades as a dozen different ailments -- including backache, stomach problems, anxiety -- and because it often gets dismissed as a touch of the common blues, true depression is far more prevalent than most people, including your family doctor, realize. Goof-offs and bumblers aren't necessarily the ones who get clobbered, either. Abraham Lincoln, J.P. Morgan, and Winston Churchill got it too. Says Jeffrey Lynn Speller, a Harvard-trained psychiatrist practicing near Boston who specializes in depressed execs: ''Often it hits the most ambitious, creative, and conscientious.'' The bad news: Depression can be a dreadful, even fatal, disease. It screws up careers and marriages. Or, as in the case of President Clinton's lawyer friend Vincent Foster Jr., it can lead to suicide. The good news: Depression is one of the most easily treated emotional ailments. More than 80% of depressives can recover, most within a few weeks, thanks to the variety of effective treatments that have become available. Doctors have a far better handle than they once did on when psychotherapy is useful and when it isn't helpful. They also have at their disposal a new generation of antidepressant drugs that come with fewer side effects, like weight gain, and are safer if taken in excess, than their predecessors. So good are these newer antidepressants, in fact, that some mental health experts worry that normal people might use them to get personality improvements of the kind described in Listening to Prozac, a current bestseller. How widespread is depression? Estimates vary, but indications are that about 15% of the U.S. population -- one man in ten and one woman in five -- will have a serious depression at some time in their lives, usually before they hit their 40s. At any given moment, about 3% of men and -- probably because of a mix of greater life stresses and subtle differences in brain chemistry -- 6% of women are depressed. Studies show that the incidence of depression has been rising sharply among people born since the 1940s. Baby-boomers and busters are three to six times more likely to report a depression than someone born at the turn of the century. Reasons for the increase are not clear, but are more than just a greater awareness of depression among young people and a willingness to admit it. Some experts theorize that successive generations have higher expectations from life and are more likely to be disappointed; others blame diminished family stability. Depression comes in different forms. Manic depression, for example, is characterized by moods that swing from wild euphoria to deep despair. About 1% of the population -- as many men as women -- suffers from this disorder. A variant is something called chronic hypomania, which is more likely to hit highly intelligent people than average ones. Hypomanics may go for years with extraordinary energy, remarkable creativity, and a talent for synthesizing seemingly disparate bits of data. If their condition changes, however, which it often does, it veers toward depression 70% of the time and toward mania most other times. Speller says half the execs he treats are hypomanics who have fallen into depression. Corporate turmoil and the flat economy are also feeding the increase in depression. More executives and professionals are seeking help than in the go- go 1980s, at least according to anecdotal evidence supplied by shrinks. This trend is supported by one of the few surveys of depression among corporate types. Some four years ago, experienced clinicians interviewed more than 1,800 managers and engineers, mostly white males in their 40s, who worked at Westinghouse Electric, a company then in upheaval. Evelyn Bromet, a psychiatry professor at the State University of New York at Stony Brook, who ran the survey, describes the results as ''astounding.'' Bromet found 23% of the people interviewed had experienced a major depression in their lives; of the 23%, nearly one out of ten had been depressed in the preceding 12 months. ''All the literature said depression was more prevalent among poorer or less educated people,'' she says. ''That was not the case.'' Scientists are increasingly persuaded that what depressed people suffer from is usually a biochemical problem, a bit like diabetes, caused by an imbalance of chemicals in the brain. The imbalance is triggered often, but not always, by stressful life events -- certainly a personal loss, but sometimes even positive news like a promotion, a baby, or a new home. A tendency to depression is not a weakness or a self-indulgence, but runs in families; indeed, it's probably a genetic trait. The parents, siblings, and children of a depressed person are four times more likely to get depressed than a nonrelative; his identical twin is at ten times the risk. Depression can clobber anybody, often out of any seeming proportion to what triggered it. Consider the case of a man in his 50s who signed on as chief financial officer at a Texas oil field services company in the midst of a takeover battle. The CEO told him to fire the controller, but the CFO fought to keep him, arguing that he needed the older man's knowledge of the company. The two men became fast friends. Then, after a brief illness, the controller died. To the new CFO's amazement, the death triggered a colossal depression. ''I was at the peak of my career. I'd never been sick a day in my life,'' he says. ''Suddenly I couldn't eat or sleep.'' His weight dropped rapidly, and he became so anxious that he found himself screaming in the car on the way to work. He barricaded himself at home on weekends, never answering the phone or reading the mail, and rarely emerged from his office at work. ''I became so burdened by the depression I could barely think. I'd stare at six pairs of identical white underwear in the morning and couldn't decide which set to put on.'' Though his symptoms were common, his case proved particularly difficult to treat. One psychiatrist experimented with a variety of antidepressant medicines for more than a year, to no avail. Then the patient switched to another doctor, who urged him to go to a special program for executives and professionals at the Menninger Clinic in Topeka, Kansas. His medication was corrected, he attended group therapy sessions with a dozen other ailing managers and professionals for about a month, and the depression lifted. It has not been back. Not surprisingly, feeling depressed, as this man did, is one of the two major ways for you to identify depression in yourself. The other principal symptom is anhedonia, a markedly diminished interest in just about everything, including pleasurable activities, that goes on nearly all day, every day. If you, or people around you, observe either of these mindsets for two weeks or more, watch out for seven other signals of possible depression. These include a big increase or loss in weight, sleeplessness or oversleeping, fatigue, slowed body movements or thoughts, feelings of worthlessness or guilt, inability to concentrate or indecisiveness, and thoughts of death or suicide. If you display any five of these symptoms, including the down mood or anhedonia, you've almost certainly got a major depression. Lesser afflictions, like burnout, grief, and sadness, often complicate the diagnosis of depression. But Dr. Donald E. Rosen, head of the executive program at Menninger, points out one big difference: ''Depression doesn't respond to common sense,'' he says. ''A depressed person takes a vacation and doesn't feel better. Or a friend tries to cheer him up, and he feels worse.'' People grieving over a deceased relative or sad about a plateauing career don't have suicidal thoughts and don't lose interest in food or sex for days at a time, says Rosen. Nor do they become seriously forgetful. Manic-depressive illness, also known as bipolar disorder, has different symptoms but can be even more disruptive. Manic-depressives may go years with normal moods. Then, in addition to the lows, there are periods of inflated self-esteem, racing thoughts, and talkativeness. Manics will sometimes go off on wildly inappropriate spending sprees, or carry on embarrassingly public love affairs. MILD CASES of manic mood can boost a career. If it stays under control, the energy and dazzle associated with chronic hypomania can propel you upward like a roman candle. Hypomania sometimes gives entrepreneurs the drive they need to build companies. But the possibility of depression, the other side of this coin, is always there. Even harder to spot is a swerve toward dysfunctionally manic behavior. One CEO decided to diversify his company by opening a series of spas in the U.S. that would use Russian techniques for restoring sexual potency to men. His board of directors was accustomed to his brilliant flashes, but even so began to push for details. The CEO became enraged at their questions and eventually quit, claiming he was surrounded by fools. The company dropped the plan. A hypomanic shifting into very high gear may not always notice what is going on, even though it has happened to him before. Gary Goldsmith, vice president of his family's industrial packaging supply company in northern New Jersey, and an amateur photographer and music buff, is a hypomanic who has identified his own early warning signs of when the disease is striking: ''I can tell the manic stuff is starting when I find I'm buying more film in a week than I've used all year. When I haven't bought a compact disk for a month, I know I'm getting depressed.'' Goldsmith relies on a blend of therapy and medication to take him out of both behavior extremes. But it took him years to arrive at that mix. Indeed, the biggest dilemma facing most newly diagnosed victims is choosing the kind of treatment they should get. Where do you go? General practitioner or shrink? Medicine or therapy? If the depression is mild, you will probably fare about equally well with antidepressant medication or short-term psychotherapy. Says Frederick K. Goodwin, director of the National Institute of Mental Health: ''Take your choice and try it for several weeks.'' An extensive guideline for doctors on treating depression, released this year by the U.S. Department of Health and Human Services, determined that medicine typically began relieving depression in four to six weeks, therapy in six to eight weeks. The difference in price may help you decide which treatment to use. Six weeks on Prozac will set you back about $100, while twice a week on the couch for that long would cost $1,200. Insurance plans generally pay for 80% of medications but only half of therapy. If you decide to go to the family doctor for help, tell her explicitly that you think you're depressed. Listing such symptoms as sleeplessness, headaches, or anxiety and hoping for the right diagnosis may not work. Goodwin says nonpsychiatrist doctors spot major depression in only 25% of cases. They are likely to prescribe an anti-anxiety drug or sleeping pill, which may act as a depressant and make matters worse. If the treatment you eventually embark on doesn't begin working in around six to eight weeks, do something. Ask to change medication, switch from therapy to medication, or find a more compatible therapist. Consider switching, too, if you're not almost completely better in 12 weeks. Dr. Donald F. Klein, director of research at the New York State Psychiatric Institute and co-author of the book Understanding Depression, cautions that even some psychiatrists aren't up to date in treating depression, being too wedded to lengthy psychoanalysis, perhaps, or unskilled in fine-tuning medication. For help in finding a doctor experienced in treating depression, contact the National Depressive and Manic-Depressive Association (800-826-3632), the National Foundation for Depressive Illness (800-239-1297), or the National Mental Health Association (800-969-6642). For moderate to severe depression, there is a growing consensus: Get medicine right away. The health department guideline determined there is almost no evidence of psychotherapy alone being useful in eliminating severe depression. Says Goodwin: ''If the symptoms are becoming relentless, if you're experiencing major sleep disturbance, if your thoughts feel like they're pushing through molasses, nobody would question using medication.'' Even people with a serious episode of depression rarely stay on drugs for long. Untreated, major depressions generally last six months to two years, and the average duration is nine months. To see if the underlying depression has ended, doctors usually try easing patients off medication after six months. If symptoms kick up again, the medicine is resumed; otherwise it is tapered off. Why use medicine? Nobody is sure exactly what causes depression, but scientists are homing in on those chemicals in the brain. Nerve cells communicate by squirting a bit of a substance, called a neurotransmitter, from the end of their tentaclelike projections. The neurotransmitter causes adjoining nerve cells to react and signal their neighbors. Meanwhile, the first cell reabsorbs and neutralizes some of its transmitter chemicals. In depressed people, there isn't enough of three transmitters that affect mood. Antidepressants seem to work by stopping the brain from reabsorbing and neutralizing the chemicals so fast. Two older classes of antidepressants, called tricyclics and MAO inhibitors, affect the reabsorption of several transmitters but cause unwanted side effects -- including dry mouth, lightheadedness when standing, and weight gain. Users of MAO inhibitors have to avoid certain foods. The newest class, by comparison, affects mainly one transmitter, called serotonin. This class of drug is known as ''selective serotonin re-uptake inhibitors,'' or SSRIs, and includes Prozac from Eli Lilly; Paxil, made by SmithKline Beecham; and Zoloft, by Pfizer. Because SSRIs are safer, have fewer side effects than older drugs, and are aggressively promoted by drug companies, their market share is growing rapidly. In the five years since SSRIs came on the market, more than ten million people worldwide have used them. Lilly sold over $1 billion worth of Prozac last year. One depression treatment still causes shivers in those who consider it: electroshock, or electroconvulsive therapy (ECT). Electrodes are placed on the scalp and a mild current is passed through them, which can trigger a brief seizure and unconsciousness. ECT is used in cases where medicine doesn't work or can't be used because of other medical conditions. While they don't know exactly how it works, doctors say it is safe and proves effective for more than half the people treated. The process got a terrible reputation 30 years ago when patients suffered memory loss and broken bones brought on by the convulsions the shocks caused. The use of anesthetics, muscle relaxants, and new ways of positioning the electrodes has greatly reduced the problems. ECT patients include Terrence Roe, a retailing consultant. He went into a steep depression while working in Italy several years ago, then had a heart attack. He returned to the U.S. for treatment, but he couldn't use antidepressants because of his heart medication. Instead, he agreed to get shock treatment at the Carrier Foundation, a private psychiatric hospital in Belle Mead, New Jersey. He was frightened at first, but now describes the treatment almost casually: ''You lie down, they give you medicine, you fall asleep, you wake up groggy a few minutes later. You go with a nurse and get a Coke, and in an hour you're fine.'' He has not been depressed since. If you are unenthusiastic about taking an antidepressant drug, know that you have company. In one survey, 70% of the population said they'd take medicine for a headache, but only 12% would touch an antidepressant. They may have been deterred by an anti-Prozac campaign by a group affiliated with the Church of Scientology, which asserted that the drug causes suicidal or violent impulses. The Scientologists' claims were refuted by the Food and Drug Administration in 1991. Studies on the major classes of antidepressants have determined all are equally effective in treating depression, though sometimes one class or brand of drug won't work on an individual while another will. None is more likely than others to trigger dangerous impulses, but judging this is complicated by the fact that self-destructive behavior is far from unknown among depressed people. On rare occasions, the risk of suicide may actually increase once medication begins, because the drug lifts the lethargy that previously protected those contemplating self-destruction, so some people are hospitalized when they first go on antidepressants. A few major misconceptions about antidepressants should be dispelled: They are not mood elevators or tranquilizers. If you are depressed, they will probably make you well. If you are not depressed, they won't make you extra- happy. Klein compares them with aspirin, which will reduce a fever to normal but won't affect a normal temperature. Antidepressants, some of which have been in widespread use for more than 20 years, are not known to be addictive and don't require steadily increasing doses to be effective. There is virtually no illegal street trafficking in such drugs. THERE IS, however, growing fascination with them, triggered in part by Listening to Prozac. The author, Peter D. Kramer, a Brown University psychiatrist, describes a handful of patients not just relieved of depression by Prozac but transformed into more radiant, attractive people than they had ever been. Though the book is mostly a meditation on what makes up a personality, it reads at times like an advertisement for Prozac. ''I had never seen a patient's social life reshaped so rapidly,'' Kramer writes in one passage. Elsewhere he frets that people may feel that social or business success will require them to improve their normal, adequate personalities with Prozac, much the way women have their breasts enlarged. An instant personality pill? No, says Kramer, who insists: ''All of these overnight transformations were seen in people who were ready because of therapy.'' NIMH director Goodwin agrees that a healthy person can't use Prozac to buff up his personality before that big presentation to the board of directors. A patient has to be depressed for the medicine to work. That said, even mental health experts cautiously allow that there is ''probably not, but maybe'' a case for trying an SSRI like Prozac even if you're not sure you're depressed. The reason, according to Goodwin: ''A person who has had all the depression symptoms in their mildest form for a year or more may come to think that that is their personality, and be surprised by the changes that result from medication.'' In short, the SSRIs are safe enough to be useful as a diagnostic tool. If you get better, you needed it. Goodwin's caution on this matter: ''I'm strongly opposed to a doctor dispensing antidepressants without taking the time to evaluate the patient's psycho-social situation.'' That means spending 45 minutes talking on first meeting and 15 minutes a week thereafter for at least a month. Even if the pills are working, don't dismiss the potential of psychotherapy. Therapy is probably useful during or after medication to repair other problems associated with a depression. A driven executive who spent a lifetime relentlessly pursuing his career is obviously a candidate for depression when he gets laid off, and medicine can help. But if all that ambition snuffed out relationships with wife, kids, and the community, eliminated hobbies, and continues to eat away at him, pills won't help enough. Therapy might. What do you do if you suspect a spouse or co-worker is depressed? Spouses generally spot the depression long before co-workers. If suggestions that a family member see a doctor are likely to backfire or be met with denial, call the doctor directly. ''About half the calls I get are from wives,'' says Speller, the Boston psychiatrist. ''They're rarely wrong.'' Speller says that he asks wives to contact a friend of the depressed person, preferably one who has been through depression himself. Then Speller, the wife, and the friend talk together to decide on a way to coax the sufferer into treatment. Co-workers, like Vince Foster's at the White House, will be slower to notice. Says Rosen at Menninger: ''Most executives wrestling with depression will have chaos at home but struggle to keep it from work.'' One woman recalls being so distraught during depressions that she banged her head against a wall until she passed out. Still, she showed up at her university job every day. ''The worse I felt, the more perfect I tried to look. I spent a lot of time organizing everything so I didn't look out of control. I was punctual. I had a perfect briefcase.'' If you are worried about a colleague at work, don't be bashful about asking how she is doing, even inquiring directly if she is down. If that's too awkward, ask the company doctor to pay a visit. The doc won't go blabbing to the CEO. If you suspect a subordinate is depressed, it may be easier to caution him that his work is suffering and gently urge him to contact the employee-assistance program without offering your diagnosis. It's no week in Hawaii, but some people who have suffered a major depression are almost glad they went through it. The woman working at the university says it forced her to uncover a traumatic event that had haunted her for years. The oil executive declares that he wouldn't go back to his old company. ''I've got too much self-confidence now. I couldn't sit still and watch the CEO screw things up.'' If ''the black dog,'' as Churchill called his moods, starts biting, don't ignore it. Like an infection, you might shake it off -- or it might do you in. But you'll get better faster if you treat it. And who knows? You might be one of the lucky ones who get that fabulous new personality.

BOX: How to tell if you're DEPRESSED

Don't confuse this serious disease with ordinary grief or disappointment. The two major signals of true depression are:

1. Feeling sad, blue, down in the dumps 2. Diminished interest in pleasurable activities, including sex

If either of these key indicators is present, also look for:

-- Significant weight loss or gain -- Sleeplessness or excessive sleeping -- Slowed body movements or thoughts -- Fatigue -- Feelings of worthlessness or guilt -- Impaired concentration, indecision, or forgetfulness -- Thoughts of death or suicide

Either one of the first two symptoms, coupled with any four of the last seven, means you probably have a major depression. If the symptoms have lasted a few weeks or if they include thoughts of suicide, see a doctor now.

CHART: NOT AVAILABLE CREDIT: FORTUNE CHART/SOURCE: SANFORD C. BERNSTEIN & CO. CAPTION: FIGHTING THE BLUES