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Depression
Depression (i.e. major depressive disorder) is a common and serious medical illness that adversely affects how you feel, think and act. It is basically a mood disorder that makes you always feel sad or feel a lack of interest in life and the activities you once enjoyed.

Depression can have many causes and risk factors, of which the more common ones include:

Depression (i.e. major depressive disorder) is a common and serious medical illness that adversely affects how you feel, think and act. It is basically a mood disorder that makes you always feel sad or feel a lack of interest in life and the activities you once enjoyed.

People suffering from depression experience symptoms related to their:

These symptoms range from mild to severe, must last at least two weeks, and must represent a change in your previous level of functioning for a confirmed diagnosis. Since most cases of depression involve thoughts of suicide or self-harm, it is important to diagnose and treat depression on time.

What is depression? – Helen M. Farrell
How To Cope With Depression
Insight Into Depression – Sadhguru