A mental illness is an illness of the mind. People with a mental illness may behave in strange ways  or have strange thoughts in their view or the view of others. Mental illnesses can affect the day to day functioning of a person. 

Mental illnesses develop during the life of a person. This may be linked to genes and experience. What is considered as a mental illness has changed over time. What is considered to be a mental illness may not be one in a different culture. The Diagnostic and Statistical Manual of Mental Disorders (DSM) by the American Psychiatric Association is used around the world. 

People with a mental illness sometimes have problems dealing with other people, or struggle with daily functioning. Treatment and certain medications can help people with daily functioning.

In some cases, mental illnesses change the way the brain works. Many conditions which affect the brain are not mental illnesses, as they do not change the way in which people think: Neither epilepsy nor Parkinson's disease is a mental illness, yet both affect the brain.

Common mental problems 
According to the New Freedom Commission on Mental Health in the United States, the most common type of disability in the United States is major mental illnesses (which include major depression, bipolar disorder, personality disorders, body dysmorphic disorder, schizophrenia, and obsessive-compulsive disorder).

Thirty-three percent of North American adults will have a mental illness in a given year, according to the National Alliance on Mental Illness.  But in more than half of these cases, the mental illness is not bad enough to disrupt daily life activities.

Partly inherited 
There is a genetic basis which makes some people more likely to develop mental illness. A study published in The Lancet, a medical journal, found the same set of genetic markers in people with five different disorders: schizophrenia, bipolar disorder, autism, major depression, and attention deficit hyperactivity disorder (ADHD).

Treatment of mental illnesses 
Mental illnesses can be treated by:

 medication
 Therapy
 Psychoeducation
 Lifestyle changes

Some more controversial treatments include
 electroconvulsive therapy
 Lobotomy

Many counties have laws that permit to treat people against their will.

Violence 
Even though the media show this differently, studies have shown that people with a severe mental illness are not necessarily violent. Statistically, violence is often caused by factors such as drug abuse, or those related to the personal, social and economic situation.

Findings show that people with a mental illness that live in the community will be victims of violence more often that they will be those starting or spreading it. A study that looked at people diagnosed with "severe mental illness" living in a US inner-city area found that a quarter of them had been victims of at least one violent crime over the course of a year; this proportion was eleven times higher than the average. For people which have been diagnosed with a mental illness it is more difficult to get a trial started, because fewer people believe them, and many people have a prejudice against people with a mental illness.

There are a few specific diagnoses which are defined by conduct problems and violence. These include conditions such as childhood conduct disorder, adult antisocial personality disorder or psychopathy. There are conflicting findings about how much specific symptoms are linked to an increase in violent behaviour. These symptoms include psychosis (hallucinations or delusions) that can occur in disorders such as schizophrenia, delusional disorder or mood disorder. The factors that lead to violent behaviour are more often demographic or economic in nature, such as being young, male, of lower socioeconomic status or of abusing drugs, including alcoholism, to which some people are particularly vulnerable.

Ableism

Some studies show that disabled people are sometimes treated badly. For example, people with mental illness may deal with ableism. An example of ableism is insults, for example calling a mentally ill person a retard.

High-profile cases have led to fears that serious crimes, such as homicide, have increased because people with certain mental illnesses are not treated in specialized institutions, but the evidence does not support this conclusion. Violence that does occur in relation to mental disorder (against the mentally ill or by the mentally ill) typically occurs in the context of complex social interactions, often in a family setting rather than between strangers. It is also an issue in health care settings and the wider community.

Related pages 
Mental state examination
Neurodiversity
Lunatic
 Common abbreviations for mental illnesses

References

Other websites 

 National Alliance for the Mentally Ill
Diseases