Mental illness is a collective term that refers to all the different types of mental conditions, including those that affect your mood, your thinking and your behavior.

What is it?

Mental illness is a collective term that refers to all the different types of mental conditions, including those that affect your mood, your thinking and your behaviour. To be classified as a mental illness, a condition must cause distress in your life and reduce your ability to function in one or more areas of your life, such as at work, in relationships or in social situations.

More than 200 conditions are classified as mental illnesses, ranging from minor to severe. Common mental illnesses include depression and schizophrenia.

A mental illness is technically considered a disorder rather than a disease because it's classified by descriptions of signs and symptoms that are open to interpretation. In general, what's considered a mental illness comes down to the severity of signs and symptoms, how long they've lasted, and how much they impair your ability to function in your daily life.


Signs and symptoms of mental illness can vary greatly depending on the particular disorder and your age, among other factors. Symptoms also can be related to emotions, thoughts (cognitive), behavior or physical issues.

Emotional, behavioral and cognitive symptoms

Emotional, behavioral and cognitive symptoms of mental illness may include:

  • Feeling sad or down
  • Confused thinking
  • Excessive fears or worries
  • Withdrawal from friends and activities
  • Problems sleeping
  • Delusions or hallucinations
  • Inability to cope with daily problems or stress
  • Alcohol or substance abuse
  • Significant changes in eating habits
  • Sex drive changes
  • Excessive anger, hostility or violence

Physical symptoms

Physical symptoms of mental illness may include:

  • Numerous unexplained physical problems
  • Fatigue
  • Back pain
  • Chest pain
  • Digestive problems
  • Dry mouth
  • Headache
  • Sweating
  • Weight gain or loss
  • Rapid heart rate
  • Dizziness

Putting symptoms into context

When exactly a symptom indicates a mental illness isn't black and white. For instance, what's considered an excessive fear, say of spiders or public speaking, can vary from person to person. And in some cultures and situations, certain behaviours or thoughts may be considered normal, while in other situations they may be considered abnormal.

In general, signs and symptoms may indicate a mental illness when they cause you distress and they interfere with your ability to function in your daily life. You may have trouble coping with emotions, stress or anger, for examples. Or you may find it difficult to handle family, work or school responsibilities.

With some types of mental illness, though, such as schizophrenia or bipolar disorder, you may not realize the extent of your problems — instead, it may be family or friends who are aware that you may have a mental illness.


There's no specific identifiable cause of mental illness. But mental illnesses, in general, are thought to be caused by a variety of biochemical, genetic and environmental factors:

  • Biochemical. Biochemical causes are related to naturally occurring processes in your body, leading some experts to characterize mental illnesses as brain disorders. This is the "nature" part of the nature vs. nurture debate. Some evidence from imaging studies indicates that people with mental illness have physical changes in their brains. The significance of these changes is still uncertain but may eventually help pinpoint causes. The naturally occurring brain chemicals called neurotransmitters, which are linked to mood, also may play a role in some mental illnesses. Hormonal imbalances also could be a culprit.
  • Genes. Your genes direct all your body's functions and define you as a living human being. Some studies suggest that mental illness is linked to certain inherited genes. In fact, mental illness is more common in people whose biological family members also have a mental illness. Researchers are trying to find genes that may be involved in causing mental illness. Some research suggests that you may have a genetic vulnerability to developing a mental illness and that your life situation may trigger the actual development of a mental illness.
  • Environment. Environment is also thought to play a causal role in some way. Environmental causes are situations in your life that are difficult to cope with, such as the loss of a loved one, financial problems and high stress. This is the "nurture" part of the nature vs. nurture debate.

Risk factors

Although the precise cause of mental illness isn't known, certain factors may increase the risk of developing or triggering mental illness, including:

  • Having other biological relatives with a mental illness
  • Malnutrition or exposure to viruses while in the womb, which is linked to schizophrenia
  • Stressful life situations, such as financial problems, a loved one's death or a divorce
  • Chronic medical conditions, such as cancer
  • Combat
  • Taking psychoactive drugs during adolescence
  • Childhood abuse or neglect
  • Lack of friendships or healthy relationships


Mental illness exacts a heavy toll. It's one of the leading causes of disability. Aside from reducing your overall quality of life, untreated mental illness can result in severe emotional, behavioral, health, and even legal and financial problems. Complications that mental illness may cause or be associated with include:

  • Suicide
  • Substance abuse
  • Homicide
  • Heart disease and other medical conditions
  • Work or school problems
  • Family conflicts
  • Relationship difficulties
  • Social isolation
  • Poverty
  • Homelessness


When doctors believe someone has a mental illness, they typically run a series of medical and psychological tests and exams. These can help rule out other problems that could be causing your symptoms, pinpoint a diagnosis and also check for any related complications. These exams and tests generally include:

  • Physical exam. This may include measuring height and weight, checking vital signs, such as heart rate, blood pressure and temperature, listening to your heart and lungs, and examining your abdomen.
  • Laboratory tests. These may include a complete blood count (CBC), screening for alcohol and drugs, and a check of your thyroid function.
  • Psychological evaluation. A doctor or mental health provider talks to you about your thoughts, feelings and behavior patterns. He or she asks about your symptoms, including when they started, how severe they are, how they affect your daily life and whether you've had similar episodes in the past. You also discuss any thoughts you may have of suicide, self-harm or harming others.

Pinpointing which mental illness you have

It sometimes can be difficult to determine which particular mental illness or mental illnesses you have. For one thing, many mental illnesses share similar symptoms. Also, a diagnosis is often based largely on how you describe your symptoms, along with how your doctor interprets those symptoms and observes you behaving. Because of this, it can take some time and effort to get an accurate diagnosis. Be sure to stick with it, though, so that you can get appropriate treatment designed for your particular illness and situation.

The symptoms and clinical features for each mental illness are detailed in a book called the Diagnostic and Statistical Manual of Mental Disorders (DSM). This manual is published by the American Psychiatric Association and is used by mental health providers to diagnose mental conditions and by insurance companies to reimburse for treatment.

To be diagnosed with a particular mental illness, you must meet the criteria for that illness listed in the current version of the DSM. Mental illnesses are grouped together in the DSM based on their symptoms. The concept of mental illness is somewhat controversial, and even experts sometimes disagree about what's considered normal or abnormal mental health. The scope of what's considered a mental illness continues to evolve.

Classes of mental illness

The main classes of mental illness are:

  • Mood disorders. These include disorders that affect how you feel, such as persistent sadness or feelings of euphoria. Among them are major depression, dysthymia and bipolar disorder.
  • Anxiety disorders. Anxiety is an emotion characterized by the anticipation of future danger or misfortune accompanied by a feeling of being ill at ease. Examples include panic disorder, obsessive-compulsive disorder, specific phobias and generalized anxiety disorder.
  • Substance-related disorders. These include problems associated with the misuse of alcohol, nicotine, caffeine and illicit drugs.
  • Psychotic disorders. These disorders impair your sense of reality. The most notable example of this is schizophrenia, although other classes of disorders can be associated with psychosis at times.
  • Cognitive disorders. These disorders affect your ability to think and reason. They include delirium, dementia and memory problems. One of the most recognizable is Alzheimer's disease.
  • Developmental disorders. This category covers a wide range of problems that usually first begin in infancy, childhood or adolescence. They include autism, attention-deficit/hyperactivity disorder and learning disabilities. But just because they're all grouped in this category doesn't necessarily mean they share a common cause or that there's a relationship among the disorders.
  • Personality disorders. A personality disorder is a lasting pattern of inner experience and behavior that is dysfunctional and leads to distress or impairment. Examples include borderline personality disorder and antisocial personality disorder.
  • Other disorders. These include disorders of impulse control, sleep, sexual functioning and eating. Also included are dissociative disorders, in which your sense of self is disrupted, and somatoform disorders, in which there are physical symptoms in the absence of a clear physical cause, such as hypochondriasis.