The World Health Organization outlined several types of mental illnesses and some warning signs to look out for:

Depression

Also known as clinical depression or major depressive disorder, this is one of the most common types of mental illness. Depression, which is also classified as a mood disorder, interferes with day-to-day living and causes severe distress to those experiencing it. Some of these symptoms or "episodes" can happen every day, nearly every day, or at least for two weeks, according to the National Institute of Mental Health (NIMH).

Warning signs:

Persistent sad, anxious, or “empty” mood
Feelings of hopelessness or pessimism
Feelings of irritability, frustration, or restlessness
Feelings of guilt, worthlessness, or helplessness
Loss of interest or pleasure in hobbies and activities
Decreased energy, fatigue, or feeling slowed down
Difficulty concentrating, remembering, or making decisions
Difficulty sleeping, waking early in the morning, or oversleeping
Changes in appetite or unplanned weight changes
Thoughts of death or suicide or suicide attempts

For more information, visit NIMH's webpage on depression.

Anxiety Disorders

Everyone has anxiety. It's natural to feel worried about things like your finances or work performance, but for some people, their anxiety can last for years and inhibit them. Anxiety disorders are characterized by excessive worry and fear capable of disrupting daily activities, including school, work, self-care, and relationships. There are many kinds of anxiety disorders, including generalized anxiety disorder, panic disorder, social anxiety disorder, certain phobias, and more.

Warning signs vary by the specific disorder. For more information about the most common ones, visit NIMH's webpage on anxiety disorders.

Eating Disorders

Most people are aware of anorexia and bulimia due to some celebrities getting candid about their struggles with these well-known illnesses. You don't have to be a model or high-profile figure to get an eating disorder, though. Millions of Americans have abnormal eating patterns and an unhealthy fixation on their weight and body image, leading to serious or even fatal effects on their body.

Since every eating disorder has distinct characteristics and warning signs, check out NIMH's page on eating disorders for the full scope.

Post-traumatic Stress Disorder (PTSD)

Post-traumatic stress disorder (PTSD) is a disease commonly associated with military veterans, but the reality is anybody can experience it. Those who go through a traumatic event in their life, whether it's a car accident, the loss of a loved one, or sexual assault -- they're at risk of developing this illness. People who have PTSD re-experience that specific trauma constantly, to the point where it interferes with daily living.

Warnings signs:

Flashbacks—reliving the trauma over and over, including physical symptoms like a racing heart or sweating
Bad dreams and difficulty sleeping
Frightening thoughts
Staying away from places, events, or objects that are reminders of the traumatic experience
Avoiding thoughts or feelings related to the traumatic event
Feeling tense or “on edge”
Having angry outbursts
Trouble remembering key features of the traumatic event
Negative thoughts about oneself or the world
Distorted feelings like guilt or blame
Loss of interest in enjoyable activities

NIMH has a page dedicated to breaking down PTSD and the steps for diagnosis.

Schizophrenia

Schizophrenia has been the subject of many misconceptions thanks to movies, TV shows, and other forms of media. This serious illness affects a person's behavior, emotions, and perception of reality. While symptoms vary from person to person, experts recommend those seek treatment after the first episode of psychosis, which includes confused speech, paranoia, strange or intense feelings, and other troubling symptoms.

Warning signs:

Hallucinations
Delusions
Unusual or illogical thoughts
Abnormal body movements
Trouble communicating thoughts or processing information
Having trouble planning and sticking with activities, such as grocery shopping
Having trouble anticipating and feeling pleasure in everyday life
Talking in a dull voice and showing limited facial expression
Avoiding social interaction or interacting in socially awkward ways
Having trouble focusing or paying attention
Having trouble anticipating and feeling pleasure in everyday life

NIMH has a more detailed overview of schizophrenia on its website.

Diagnosis and Treatment

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