Depression is one of the most common mood disorders in the United States. Roughly 1 in 10 adults — that's upwards of 22 million Americans — experience depression at any given time.

Depression is a complex condition, and experts aren't entirely sure what causes it. However, research suggests that various genetic, biological, environmental, and psychological factors play a role in depression, including:

• Family history
  Depression is more common in people whose blood relatives have also had it. In fact, a family history of depression can increase your risk by three to four times.
• Chronic diseases, such as cancer or diabetes
  Depression can occur with other illnesses, such as cancer and diabetes. Depression can make these illnesses worse, and vise versa. Furthermore, medication taken for various diseases may cause side effects that contribute to depressive symptoms.
• Gender
  ‍Women are twice as likely to be diagnosed with depression as men. While the reason for this gender gap remains unclear, hormonal changes that women experience at different points in their lives may play a role.
• Major life events
  Stressful life events, including marital strain, divorce, health problems, and financial challenges, can contribute to the risk of developing clinical depression.‍
• Difficulty in childhood
  ‍Environmental factors also contribute to the development of depression. Exposure to physical, sexual, or emotional abuse, a stressful home environment, and poor relationships with your parents can make you more vulnerable to depression later in life.
  

Depression is not a one-size-fits-all diagnosis. There are many types, each with different causes, symptoms, and treatments. The most common forms of depression include:

• ‍Major Depressive Disorder (MDD)
  ‍Major Depressive Disorder — also called clinical depression — is a mood disorder that causes persistent feelings of sadness. If you experience depression symptoms most of the day, nearly every day for two weeks or more, you may have MDD.
• Persistent Depressive Disorder (Dysthymia)
  ‍While less severe than MDD, Persistent Depressive Disorder is — as the name suggests — persistent. Typically, it lasts for two years or longer. People with Dysthymia may feel like they’re constantly going in and out of depression.‍
• Perinatal depression
  ‍Perinatal depression affects new and expectant mothers. The term includes depression during pregnancy (prenatal depression) and depression after delivery (postpartum depression). Perinatal depression can cause extreme fatigue, anxiety, mood swings, anger, and a loss of interest in activities.
• ‍Premenstrual dysphoric disorder (PMDD)
  ‍Premenstrual dysphoric disorder is a form of depression that affects people who get periods. PMDD can cause severe irritability, depression, or anxiety. Symptoms usually show up the week before you start your period and last until a few days after it begins.
• ‍Seasonal affective disorder (SAD)
  ‍The aptly named SAD is a form of depression that comes and goes with the seasons. Usually, it starts in the fall and winter months and goes away during the spring and summer. People with SAD may experience depression on a recurring, seasonal basis — but feel mentally healthy the rest of the year.‍
• Bipolar disorder
  ‍Bipolar disorder is a mental health condition that causes extreme shifts in mood. Though different from depression, people with bipolar disorder often experience periods of low moods that match the symptoms of depression. However, they also experience mania: periods of high or euphoric moods.

Depression symptoms can vary widely from person to person. For example, some people with depression have trouble sleeping, whereas others may want to sleep all the time. Also, different forms of depression have distinct symptoms and varying levels of severity.

While depression looks different for everyone, common symptoms include:

• A persistent sad, anxious, or "empty" mood
• Feelings of hopelessness, guilt, worthlessness, or helplessness 
• Loss of interest in daily life, including hobbies or activities
• Fatigue, or feeling "slowed down"
• Difficulty concentrating, remembering, or making decisions
• Trouble sleeping, either difficulty falling asleep, waking up in the middle of the night, or oversleeping 
• Changes in appetite and weight 
• Thoughts of suicide or suicide attempts 
• Irritability
• Aches, pains, headaches, cramps, or digestive problems that don't have a clear physical cause