updated 10/11/2004 4:00:01 PM ET 2004-10-11T20:00:01

Depression is a serious mental illness that can significantly interfere with a person’s ability to function. It affects up to 2.5 percent of children and about 8 percent of teens in the United States, according to one federal study.

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The most severe types of clinical depression are called major depressive disorder (MDD) and bipolar depression, the depressed phase of bipolar disorder. Symptoms for MDD include a depressed mood or loss of interest in everyday activities that lasts nearly every day for at least two weeks. Symptoms also can range from long-standing feelings of anxiousness and irritability to sustained periods of sleeping and eating too much or too little.

Alternatively, people with bipolar disorder fluctuate between major depression, similar to MDD, and extreme highs or “mania” that can lead to aggressiveness or delusional behavior — anything from unwise business decisions to a spate of sexual encounters.

Excessive alcohol use or drug abuse is also prevalent among those with underlying depressive disorders. And both types of depression can lead to suicide.

Some of the warning signs of suicide are:

—Talking about suicide, including a method;

—Talking about hopelessness and worthlessness;

—Suddenly being happier and calmer;

—Making unusual visits or calling people one cares about;

—Making arrangements, putting affairs in order;

—Giving things away.

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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