how it is caused?
Childhood abuse, either physical, sexual or psychological are all risk factors
Major depressive disorder
Childhood abuse, either physical, sexual or psychological, are all risk factors for depression, among other psychiatric issues that co-occur such as anxiety and drug abuse.
what does single episode major depressive disorder mean UNANSWERABLE what are symptoms of  this disorder? Low mood, symptoms of psychosis; like delusions and hallucinations are some of them
wiki:1591159
Major depressive disorder [SEP] Cause
The biopsychosocial model proposes that biological, psychological, and social factors all play a role in causing depression. The diathesis–stress model specifies that depression results when a preexisting vulnerability, or diathesis, is activated by stressful life events. The preexisting vulnerability can be either genetic, implying an interaction between nature and nurture, or schematic, resulting from views of the world learned in childhood. Childhood abuse, either physical, sexual or psychological, are all risk factors for depression, among other psychiatric issues that co-occur such as anxiety and drug abuse. Childhood trauma also correlates with severity of depression, lack of response to treatment and length of illness.