Most people feel anxious or depressed at times. Losing a loved one, getting fired from a job, going through a divorce, and other difficult situations can lead a person to feel sad, lonely, scared, nervous, or anxious. These feelings are normal reactions to life's stressors.
But some people experience these feelings daily or nearly daily for no apparent reason, making it difficult to carry on with normal, everyday functioning. These people may have an anxiety disorder, depression, or both.
It is not uncommon for someone with an anxiety disorder to also suffer from depression or vice versa. Nearly one-half of those diagnosed with depression are also diagnosed with an anxiety disorder. The good news is that these disorders are both treatable, separately and together.
Read on to find out more about the co-occurrence of anxiety and depression1,2 and how they can be treated.
Depression is a condition in which a person feels discouraged, sad, hopeless, unmotivated, or disinterested in life in general. When these feelings last for a short period of time, it may be a case of "the blues."
But when such feelings last for more than two weeks and when the feelings interfere with daily activities such as taking care of family, spending time with friends, or going to work or school, it's likely a major depressive episode.
Major depression is a treatable illness that affects the way a person thinks, feels, behaves, and functions. At any point in time, 3 to 5 percent of people suffer from major depression; the lifetime risk is about 17 percent.
Three main types of depressive disorders—major depression, dysthymia, and bipolar disorder—can occur with any of the anxiety disorders.
Major depression involves at least five of these symptoms  for a two-week period. Such an episode is disabling and will interfere with the ability to work, study, eat, and sleep. Major depressive episodes may occur once or twice in a lifetime, or they may re-occur frequently. They may also take place spontaneously, during or after the death of a loved one, a romantic breakup, a medical illness, or other life event.
Some people with major depression may feel that life is not worth living and some will attempt to end their lives.
Persistent depressive disorder, or PDD, (formerly called dysthymia) is a form of depression that usually continues for at least two years. Although it is less severe than major depression, It involves the same symptoms  as major depression, mainly low energy, poor appetite or overeating, and insomnia or oversleeping. It can manifest as stress, irritability, and mild anhedonia, which is the inability to derive pleasure from most activities.
People with PDD might be thought of as always seeing the glass as half empty.
Bipolar disorder,  once called manic-depression, is characterized by a mood cycle that shifts from severe highs (mania) or mild highs (hypomania) to severe lows (depression).
During the manic phase, a person may experience abnormal or excessive elation, irritability, a decreased need for sleep, grandiose notions, increased talking, racing thoughts, increased sexual desire, markedly increased energy, poor judgment, and inappropriate social behavior.
During the depressive phase, a person experiences the same symptoms  as would a sufferer of major depression. Mood swings from manic to depressive are often gradual, although occasionally they can occur abruptly. Learn more about bipolar disorder. 
Depression and anxiety disorders are different, but people with depression often experience symptoms similar to those of an anxiety disorder, such as nervousness, irritability, and problems sleeping and concentrating. But each disorder has its own causes and its own emotional and behavioral symptoms.
Many people who develop depression have a history of an anxiety disorder earlier in life. There is no evidence one disorder causes the other, but there is clear evidence that many people suffer from both disorders.
1. Barbee, J. G. (1998). Mixed symptoms and syndromes of anxiety and depression: Diagnostic, prognostic, and etiologic issues. Annals of Clinical Psychiatry, 10:15–29.
2. Regier, D. A., Rae, D. S., Narrow, W. E., Kaelber, C. T., & Schatzberg, A. F. (1998). Prevalence of anxiety disorders and their comorbidity with mood and addictive disorders. British Journal of Psychiatry. Supplement, 34: 24–28.