Left untreated, depression can lead to serious health complications, including putting your life at risk. Fortunately, there are effective treatments for depression through options like therapy, medication, diet, and exercise.

Sadness and grief are normal human emotions. We all have those feelings from time to time but they usually go away within a few days. Major depression , or major depressive disorder, however, is something more. It’s a diagnosable condition that’s classified as a mood disorder and can bring about long-lasting symptoms such as overwhelming sadness, low energy, loss of appetite, and a lack of interest in things that used to bring pleasure.

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Specific circumstances can trigger other forms of depression or subsets of the condition.

Major depressive disorder

It’s estimated that 16.2 million adults in the United States, or 6.7 percent of American adults, have had at least one major depressive episode in a given year.

Persistent depressive disorder

You may have a single bout of major depression or you can have recurring episodes. Persistent depressive disorder, or dysthymia, is chronic low-level depression that’s lower in severity than major depression and lasts two years or longer. These ongoing feelings of deep sadness and hopelessness, in addition to other symptoms such as low energy and indecision, occurs in 1.5 percent of U.S. adults in a given year. It is more prevalent in women than men, and half of all cases are considered serious.

Bipolar disorder

Another type of depression is bipolar disorder, or manic-depressive disorder and affects about 2.8 percent of the U.S. population in a given year. It occurs equally in men and women, while 83 percent of cases are considered severe.

The disorder involves the development of a manic, or energized mood, episode. Sometimes, these may be preceded or followed by episodes of depression. The presence of these episodes is what determines which type of bipolar disorder is diagnosed.

Seasonal depression

If you have major depressive disorder with seasonal pattern, also known as seasonal affective disorder, your mood is affected by seasonal changes. The condition occurs in up to 5 percent of the U.S. population in a given year. Seasonal depression is typically triggered by the onset of autumn and lasts throughout the winter and it very rarely occurs in summer and spring.

Geography and distance from the equator play significant roles in this disorder. Women also represent 4 out of 5 people with the condition.

Postpartum depression

As much as 80 percent of new mothers experience the “baby blues” and symptoms include mood swings, sadness, and fatigue. These feelings usually pass within a week or two.

It’s caused by hormonal changes following childbirth, lack of sleep, and the pressures of taking care of a new baby. When these symptoms persist longer than a couple weeks and severity escalates, it may be a sign of a major depressive disorder with peripartum onset, also known as postpartum depression.

Additional symptoms include withdrawal, lack of appetite, and a negative train of thought. According to the American Psychological Association, about 10 to 15 percent of U.S. women have a depressive episode within three months of childbirth. One in five new mothers experience minor depressive episodes, and as many as 10 percent of new fathers may experience this condition, too.

Dr. Christina Hibbert, award-winning author and clinical psychologist, calls this “a familial disease.” Left untreated, it can be dangerous for parents and baby.

Psychotic depression

When major depression or bipolar disorder are accompanied by hallucinations, delusions, or paranoia, it’s called major depressive disorder with psychotic features. About 25 percent of patients who are admitted to a hospital due to depression actually have psychotic depression. 1 in 13 people worldwide will experience a psychotic episode before age 75.

Prevalence of depression

The National Institute of Mental Health (NIMH) estimates that 16.2 million U.S. adults had at least one major depressive episode in 2016. This represents 6.7 percent of the U.S. adult population.

Depression is most common in ages 18 to 25 (10.9 percent) and in individuals belonging to two or more races (10.5 percent). Women are twice as likely as men to have had a depressive episode, according to the NIMH and the World Health Organization (WHO) . From 2013 to 2016, 10.4 percent of women were found to have depression, compared with 5.5 percent of men, according to the CDC .

The WHO estimates that more than 300 million people worldwide suffer from depression. It’s also the world’s leading cause of disability.