If you have a bad day and feel a little down, those feelings usually pass.

But that’s not the case all of the time.

Sometimes feelings of despair can last days, weeks or, in many cases, years. Daily activities like working, eating and sleeping can seem tedious.

Nearly 10 million Americans suffer from some sort of clinical depression in a six-month period — making depression one of the most common illnesses today.

Dr. Shelia Marcus, clinical assistant professor and director of adult outpatient psychiatry at the University of Michigan Medical Center in Ann Arbor, said depression is more prevalent than most people think.

"This is a common disorder," she said.

About 12 percent of men and 20 percent to 23 percent of woman suffer from some form of depression.

Approximately 9.5 percent of the population will suffer from depressive illness during their lives.

Heinz Prechter was one of those people.

The Downriver industrialist took his life Friday after suffering from depression for nearly 30 years.

"Heinz Prechter highlights how depression can devastate family, friends, businesses and others," Marcus said.

But many people misunderstand depression, how serious it really is and what depression is and isn’t, she said.

"It is an illness," she said. "It has to do with a person who is genetically vulnerable, and oftentimes there are stressful life events that precipitate an event."

Marcus added that there are many things that depression is not.

"It is not a sign of personal weakness," she said. "It is not a kind of thing someone can pull themselves out of. It requires appropriate treatment with medication and psychotherapy."

According to the National Institute of Mental Health, many people still believe that the emotional symptoms caused by depression are "not real" and a person can shake off symptoms.

Because of these myths, many people suffering from depression either may not recognize that they have a treatable disorder or may be discouraged from seeking or staying in treatment due to feelings of shame and stigma.

Often, untreated depression is associated with suicide.

Many people with depression usually suffer from a few or several of the following symptoms:

• Persistent sadness, anxiety or "empty" moods.
• Feelings of hopelessness or pessimism.
• Feelings of guilt, worthlessness or helplessness.
• Loss of interest or pleasure in hobbies and activities that were once enjoyable.
• Decreased energy, fatigue or feeling "slowed down."
• Difficulty concentrating, remembering or making decisions.
• Insomnia, early morning awakening or oversleeping.
• Appetite and/or weight loss or overeating and weight gain.
• Thoughts of death or suicide; suicide attempts.
• Restlessness and/or irritability.
• Persistent physical symptoms that do not respond to treatment, such as headaches, digestive disorders or chronic pain.

There is no one single cause of major depression.

Psychological, biological and environmental factors all may contribute to its development, but scientific research has firmly established that major depression is a brain disorder.

Researchers also have found evidence of a genetic predisposition to major depression.

One of the most important ways to help someone suffering from the illness is a strong support system of family and friends.

Marcus said those with depression need support just the same as a person who is suffering from a physical illness would.

She said family members can be supportive in a number of ways.

First, families can support and reassure their loved ones with communication and empathy.

Marcus also suggested that family members encourage those with depression to stay in or to get treatment and to work with the providers.

Family members also can "pick up some of the slack" for the person, just like if there’s a physical illness, she said. But being sensitive about the balance of helping and allowing the depressed person to do things is critical.

Marcus said family members also are an important sense of "reality checks" for those with depression.

She said depressed people often believe they’re not good people and have a negative, hopeless stance. Families can help instill hope and offer help.

Marcus said there are a variety of medications as well as a number of cognitive behavioral and interpersonal therapies to treat depression.

Between 80 percent and 90 percent of those suffering from serious depression can be treated and return to their normal routines.

For more information on depression, visit the National Alliance for the Mentally Ill Web site at www.nami.org or the NIMH Web site, www.nimh.nih.gov. The National Depressive and Manic-Depressive Association’s Web site is www.ndmda.org.