Depression (also called major depressive disorder, unipolar depression or clinical depression) is a mental illness. Many people think that depression just means a person is very sad. However, depression can cause many symptoms in the body as well as mood problems.

Diagnosis

The ICD-10 is used around the world to diagnose people with illnesses like depression. According to the ICD-10, for a person to be diagnosed with depression, their symptoms have to last for at least two weeks. The symptoms must happen every day, or almost every day. These symptoms also have to cause problems in a person's life (like their work life, family life, social life, or other important parts of their life).

Usually, for depression to be diagnosed, a person must have five or more of these symptoms:

 Depressed mood for most of the day (feelings of sadness, emptiness, and/or hopelessness)
 Teenagers may be irritable (they may get angry easily) instead of seeming depressed
 Feeling much less interested than usual in all, or almost all, activities; or not getting any pleasure from activities
 Significant weight loss (when not dieting) or weight gain (generally a change of 5% or more in body weight)
 Sleeping more than usual, or having trouble sleeping
 Moving around more than usual (psychomotor agitation) or moving more slowly than usual (psychomotor retardation)
 Feeling tired or not having energy, nearly every day
 Feeling worthless or guilty
 Not being able to think, concentrate, or make decisions normally
 Thinking a lot about death (not just being afraid to die)

Most people who have not had depression do not completely understand its effects. Instead, they see it as simply being sad. Since it is not understood, many people criticize people with depression for not helping themselves.

Some people with depression die by suicide. Depression is one of the mental illnesses that can cause a person to have suicidal thoughts.

Prevalence

It is impossible to get an exact number of how many people have depression. There are many reasons for this. For example:

 People may not admit to having depression, because of the stigma about depression
 Some people may not be correctly diagnosed with depression
 Different doctors and mental health workers may diagnose depression differently
 The number of people diagnosed with depression is different in different cultures, and among men and women

In different cultures

Major depression is one of the most common mental illnesses in the United States. In 2014, 15.7 million adults in the United States had at least one episode of major depression. This is about 6.7% of all adults in the United States.

Around the world, depression causes more disability than any other mental health or behavioral illness, according to the World Health Organization.

However, the prevalence of depression is different in different cultures and countries.

Prince (1968) found virtually no depression in most Asian and African countries. However, psychologists such as Zhang and Kleinman have found disorders with similar symptoms, but different names. For example, in the 1980s, Zhang found that in China, there was little mention of depression. However, instead, a disorder called Neurasthenia was far more common. When interviewing Neurasthenia patients, Zhang found that their symptoms matched the symptoms of depression by over 80%. While the term "depression" was practically unheard of in China, Neurasthenia could well have been a similar disorder – or the same thing.

It is also important to take in different cultures ways of classifying depression. For example, in China, many people believe doctors solve physical symptoms, not mental problems. This may have made it more likely for "depression" to be diagnosed as a medical condition like "Neurasthenia."

In men and women

Typically, in most Western cultures, women are diagnosed with depression twice as often as men. However, men are often more likely than women to die by suicide.

In children
Depression in children can be harder to see. Children who are depressed may have a loss of appetite, meaning that they do not want to eat. They may also be clearly having more trouble in everyday life than before. For example, they may have sleep problems like nightmares; new problems with behaviour or grades at school; or be more irritable than usual.

Types of depression 

Major depressive disorder is also referred to as major, biochemical, clinical, endogenous, or biological depression. It may also be called unipolar affective disorder.

There are many subtypes of depression:
 Melancholia, or melancholic depression, is very severe (bad). It can cause many physical changes, like sleep and appetite changes. It can also cause a person to change their behavior - for example, by not wanting to be around other people.
 Psychotic depression is much like melancholia, but with hallucinations or delusions.
 Atypical depression causes anxiety and panic attacks.
 Chronic dysthymic disorder is a long-term, mild depression that lasts for at least two years. It often begins in adolescence and lasts for many years.
 Seasonal affective disorder (SAD) is when the depression seems to be related to the winter season. It usually starts in the autumn and goes away in the spring every year.  It is common in Alaska, Canada, and Scandinavia (Sweden, Norway, Denmark, and Finland), but not in Iceland.

Causes of depression 
As of 2016, scientists believe there is no one cause of depression. There is a lot of argument over whether depression is caused by biological, cognitive, or sociocultural factors:
 Biological explanations of depression focus on changes in brain chemicals (neurotransmitters).
 Cognitive explanations of depression focus on how the way we think (cognition) affects our mood. These explanations say that if a person thinks negatively, this thinking can make them feel negatively about themselves and the world around them.
 Sociocultural explanations of depression blame stressful things like divorce, losing a job, bullying, and poverty for causing depression.

The United States National Institutes of Health say that depression comes from the brain. However, scientists are still trying to find out exactly why it happens.

Possible causes
There are many theories about what causes depression.  These theories include:
 Heredity (someone's genes): Depression often runs in families. However, people can get depression even if they are not related to anyone else who has it.
 Physiology: The amounts of some chemicals in the brain. Serotonin is the main chemical in the brain that has to do with happiness. Many antidepressant medications work by balancing the amount of serotonin in the brain.
 Psychological factors: Low self-esteem and harmful thinking
 Early experiences: Events such as the death of a parent or guardian, abandonment or rejection, neglect, chronic illness, and severe physical, psychological, or sexual abuse.
 Life experiences: Losing a job, homelessness, problems with money, death of a loved one, relationship problems, separation from a partner, being a victim of crime, or other life problems can cause depression.
 Medical conditions: Some illnesses, such as hypothyroidism, hepatitis, mononucleosis, or head injuries, may contribute to depression.
 Alcohol and other drugs: Alcohol can make people depressed. If a person abuses alcohol, benzodiazepine-based tranquillizers, sleeping medications, or narcotics, it can affect how long their depression lasts and how bad it is. Some medicines like birth control pills and steroids may also contribute to depression.
 Pesticides: Several scientific studies  have found statistical links between depression and some pesticides.

Some depressed people also have other mental disorders, such as personality disorders and anxiety disorders.

Treatment 
Depression is usually treated with a combination of medication and other therapy. Good exercise helps deal with depression, since exercise releases chemicals that put a person in a better mood. Having a supportive group of friends and doing outside activities can also help prevent or ease depression.

Medication 

There are many medications that can help with depression. Many of these are called antidepressants. Examples of medications used to treat depression include:

Selective serotonin reuptake inhibitors (SSRIs). These are now the most commonly used type of antidepressant medicine. These drugs work by allowing the brain to have more serotonin. They have fewer side effects than older antidepressants. Examples include fluoxetine (Prozac), paroxetine (Paxil), and sertraline (Zoloft).
Tricyclic antidepressants. These are an older type of antidepressant. They are not used often today because they have many bad side effects and do not work as well as SSRIs.  An example is nortriptyline (Allegron).
Monoamine oxidase inhibitors (MAOIs). They may be used if other antidepressant medications do not work well. This kind of medicine cannot be taken with many different foods and other medications.

Sometimes, antidepressant medicine works better when it is used together with another drug that is not an antidepressant. These "augmentor" drugs include:

Tranquillizers and sedatives. These can help people sleep better and feel less anxious.
Antipsychotics. These can help keep stop mood swings. They can also make hallucinations better, or make them go away completely.
Lithium. This is a mood stabilizer often used for bipolar disorder, but it can also help people with depression.

If people with depression do not take their medicine the right way, the depression can get worse. A doctor must help when they want to change to another medication, or to take a different amount of a medication than before.

Psychotherapy 
Psychotherapy is often a helpful treatment for depression. In psychotherapy, a therapist helps a person to understand and solve problems which cause depression. The therapist also helps the person learn skills for dealing with their depression. 

Psychotherapy can help a person make changes in the way they think, in order to help with life problems and understand what makes depression worse. The most effective psychotherapy for depression is Cognitive behavioral therapy (CBT). This teaches a person to think in a more rational, positive, realistic manner.

Surgeries and procedures 
Electroconvulsive therapy (ECT), also called electroshock therapy or shock therapy, is used to treat a small percentage of severely depressed people. ECT uses a small amount of electricity to cause an epileptic seizure while the patient is under anesthesia. This may cause some memory loss (amnesia) or other side effects.

In the past, doctors used different ways of treating depression. These are rarely used any more. They include:
 Insulin shock treatment. The doctor would lower a patient's blood sugar so much that the patient would have a seizure.
 Lobotomy. This is a form of surgery. The surgeon would use an ice pick or a surgical knife to cut nerves in the frontal lobe of the brain.

Pet therapy  
Pet therapy and animal companionship can help people suffering from depression or anxiety.

Related pages
 Dysthymia, a long-lasting mild depression
 Mania
 Bipolar disorder, a mood disorder which involves both mania and depression
 Cyclothymia, a mild form of bipolar disorder
 Melancholia

Further reading
 Study showing genetic links between depression and four other major psychiatric disorders: autism ADHD, bipolar disorder, and schizophrenia
5 Disorders Share Genetic Risk Factors, Study Finds (The New York Times, February 28, 2013)
Psychiatric Disorders Linked Genetically (The Wall Street Journal, February 27, 2013)

References

Books 
 Books by psychologists/psychiatrists:
Beck, A. T., Rush, A. J., Shaw, B. F., Emery, G. (1987). Cognitive therapy of depression. New York: Guilford.
Klein, D. F., & Wender, P. H. (1993). Understanding depression: A complete guide to its diagnosis and treatment. New York: Oxford University Press.
Weissman, M. M., Markowitz, J. C., & Klerman, G. L. (2000). Comprehensive guide to interpersonal psychotherapy. New York: Basic Books.
Books by persons suffering or having suffered from depression:
Smith, Jeffery (2001). Where the roots reach for water: A personal and natural history of melancholia. New York: North Point Press.
Solomon, Andrew (2001). The noonday demon: An atlas of depression. New York: Sribner.
Styron, William (1992). Darkness visible: A memoir of madness. New York: Vintage Books/Random House.
Wolpert, Lewis (2001). Malignant sadness: The anatomy of depression. London: Faber and Faber.
 Self-help (bibliotherapeutic) books:
Lewinsohn, P. M., Munoz, R. F, Youngren, M. A., Zeiss, A. M. (1992). Control your depression. New York: Fireside/Simon&Schuster.

Other websites 
Clinical-Depression
Internet Mental Health: Major Depressive Disorder
 Depression Alliance website (UK charity) [useful information]
Depression Fact Sheets [Facts about the causes, symptoms, and treatment of depressive disorders.]
 Detailed information [concerning Emil Kraepelin, who identified Manic Depression]
 Depression – A Silent Killer! [Major Causes, Types of Depression, Treatment for Depression]

Mood disorders