(CNN) - For 14-year-old Ben Garnette, the backyard treehouse he and a friend built in the Virginia countryside is one of the few places he can find any peace. "When I'm bored I'll start doing stuff that I shouldn't be doing," he says. "I'll go outside and cause trouble or..get in a fight with somebody."

Ben is diagnosed with attention deficit disorder, obsessive compulsive disorder, and manic-depression. He's been in and out of trouble with the law -- breaking and entering, destroying property and discharging a firearm. His adoptive mother, Lindy Garnette, is a social worker who directs children's programs at the National Mental Health Association. She believes depression is at the root of all his problems.

She says it was the depression that caused them to seek medical help for Ben. "When he was like four or five and he would be going to bed at night he would make comments like the playground's a lonely place, you'd be better off if I'd never been born."

Ben says that he tends to replace fear or sadness with anger. "I can be fine, you know calm, patient with everybody one second and the next second I'll be throwing chairs across the room or breaking windows or punching doors."

Researchers say depression is going to be one of the world's biggest health problems in the next century. A World Health Organization study predicts by the year 2020, depression will be second only to heart disease as a cause of disability and premature death. In the United States alone, 19 million people suffer from the disease every year. Two-thirds of them go untreated.

With depression affecting some 10 percent of the American population and cited as the reason behind most suicides, National Institute of Mental Health director Dr. Steven Hyman says it would be fair to say the illness had now assumed epidemic proportions.

"There are 30,000 suicides in the United States every year and less than 20,000 homicides." says Dr. Hyman. "There are more suicides than there are deaths from many many well known diseases including AIDS."

A psychiatrist by training, Dr. Hyman says it is puzzling to him and other scientists that depression is affecting more and more people -- and at a younger age. "We've rounded up all the usual suspects, you know whether changes in family structure for example, or increases in the availability of illegal drugs, or use of alcohol, for example, might explain some of this....Unfortunately, there is no easy or straightforward explanation for why this is happening."

In adolescents, the signs of depression are not always obvious. Dr. Hyman says there's reason to believe that the stereotype of the depressed person being someone who is withdrawn and inactive may not always be correct, especially among adolescents.

"A lot of depressed adolescents can become very , very irritable and can become involved in violence. "

While scientists have discovered a genetic link to depression, others are also looking into brain chemistry for clues to depression. Brain cells communicate with each other by releasing chemicals across the synapse -- the space between the cells. When that's disrupted, disorders like depression can occur.

Drug companies and researchers are trying to find new drugs to restore the brain's chemical balance -- ones that work more quickly and with fewer side effects than the ones available now. "We think that understanding the genes that create risks will help us develop treatments that really are novel," says Dr. Hyman.

New imaging techniques show that the brain of a depressed person is organically different from the brain of a non-depressed person. The brain actually changes in depression. And new studies indicate there may be actual brain damage -- something like scar tissue -- in untreated cases of long-term depression.

From the ancient herbal remedy St. John's Wort to a pacemaker-like device that sends electrical shocks to the brain, there are efforts on many fronts to see what might work.

Some researchers are mapping the circuitry of the brain and growing brain cells on microchips. They want to implant chips in the brain which might replace the faulty electro-chemical systems of people with depression.

But even the most optimistic are skeptical there will ever be a cure. Ben Garnette has been on medications since he was seven. His mother says psychotherapy has not worked. She's concerned about his future particularly after the shootings at Columbine High School in Colorado.

Garnette says she was disturbed when she learned that one of the boys accused of shooting the children at Columbine was on Luvox because it is a drug that Ben is taking.

"I'm scared to death," she says. "When I look down the road there are days that I look at my son that he looks like everybody else's kid and I'm real optimistic... (and then) there are days and sometimes within the same day when I look at that same young man and I'm terrified because what I see is, is he going to spend his life in prison."