One in every five Americans will suffer depressive or manic-depressive illness at some time in their lives. Unlike people with many other diseases, patients with depressive illness typically get help only from their families, from the medical community or from a diverse group of small organizations.

But a new national organization, geared to helping these patients and their families, was launched this week. The nonprofit National Depressive and Manic-Depressive Association (NDMDA) is the country's first mental health advocacy group founded by patients that focuses on a single illness. NDMDA comprises 61 groups throughout the United States and Canada.

The organization plans to publish a national newsletter, develop a nationwide network of local chapters and establish a new national clearinghouse "to distribute the most up-to-date information available" on depression.

Another goal of NDMDA, founders say, is to eradicate the stigma associated with mental illness, in part by educating the public about early identification, diagnosis and proper treatment.

*To familiarize people with the disease, NDMDA offers the following warning signs of depression:

*Feelings of sadness, anxiety and hopelessness.

*Sense of impending doom or disaster.

*Reduced enjoyment and pleasure -- void of emotions.

*Loss of energy and motivation.

*Low self-esteem or guilt; feelings of worthlessness.

*Eating disturbance -- increased or decreased appetite.

*Sleeping disturbance -- increased or decreased.

*Diminished ability to think or concentrate.

*Recurrent thoughts of death, or suicidal thoughts or attempts.

The signs of mania include:

*Rapid, unpredictable emotional changes.

*Extreme irritability.

*High energy level.

*Minima sleeping.

*Racing thoughts.

*Feelings of grandiosity, inflated self-esteem.

*Distractability, i.e., attention easily drawn to unimportant or irrelevent external stimulae; excessive involvement in activities that have high potential for painful consequences which are not recognized.

For further information, write to NDMDA, Merchandise Mart, Box 3395, Chicago, Ill. 60654.