We all feel grief after a loss or get the blues now and then. But depression -- sometimes called clinical or major depression -- is different. Depression affects your moods, thoughts, body and behavior -- often for a long period of time. This condition can affect how you eat and sleep and how you feel about yourself and those around you.

Depression is roughly twice as common among women as men. Symptoms include a sad mood or a loss of interest or pleasure in your usual activities. If you are depressed, you may feel like you are "living under a cloud." You may want to withdraw from other people. You may find concentrating difficult. Everyday tasks may leave you tired. And any extra demand --such as a flat tire or a sick child -- may feel overwhelming.

Sometimes, episodes of depression are triggered by a stressful event, like a financial problem, a difficult relationship, losing a job, or the death of a loved one. But depressive episodes can occur for no apparent reason. Depression may develop slowly, over days, or weeks. Or, it can come on suddenly.

Some types of depression run in families. But you may also experience depression even if you have no family history of depression. Your own psychological makeup plays a role in whether you are likely to suffer from depression. People who have low self-esteem, who are pessimistic, or who are easily overwhelmed by stress are more likely to become depressed. Becoming depressed doesn’t mean that you’re weak; it simply means you have an illness, and a rather common one at that. Some 18 million people in the U.S. suffer from depression, making it the most common form of mental illness.